Documentation shows that the English word tender was born in the 13th century, and it immediately experienced a semantic growth spurt. Many of its early senses are obsolete now, but people still care about the word and foster its growth in the language. In fact, it developed a new sense relatively recently—in the late 20th century.
The word is a child of tendre, an Anglo-French adjective that denotes softness, delicacy, or love. Tendre is also a French verb with the meaning "to offer" or "to stretch or hold out," and English speakers and writers have fully embraced both words—first adopting tendre as an adjective in its native senses and then as a noun and verb referring to the offering of something to another, such as money or assistance.
As an adjective, tender originally specified things having a soft or delicate texture or consistency, like freshly baked bread, porcelain, or limestone. American lexicographer Noah Webster (Dad) took note that people also called a particularly soft, juicy cut of meat from the loin of an animal tender and that they referred to such succulent cuts as tenderloin. He was the first to define that word, as "a tender part of flesh in the hind quarter of beef," in his 1828 An American Dictionary of the English Language. Later in the century, tenderloin was cooked up as a name for a district of a city devoted to vice of one form or another that invited political and police corruption—basically, such districts tempted politicians and police to partake in juicy servings of graft.
[Alexander] "Clubber" Williams didn't invent police corruption and brutality, but transformed both into fine arts. In 1876, when William's superiors transferred him from a mundane East 20s precinct to the West Side's Central Broadway District, hub of Manhattan's gambling … and liquor trades, his greedy heart leaped with joy. "I've had nothing but chuck steak for a long time," Williams chortled, "and now I'm going to get a little of the Tenderloin." Previously, the precinct was "Satan's Circus," forever afterward—the "Tenderloin."
— David Pietrusza, Rothstein, 2003
(The etymological consensus is that tender in "chicken tender" is a shortening of tenderloin. That tender was first served up some time in the late 20th century.)
In the 13th century, tender began being used to describe people of a weak, fragile, or delicate constitution—specifically, the elderly or sickly—who need some TLC. (That acronym, standing for "Tender Loving Care," has been traced back to a 1960 dictionary for medical secretaries.) The adjective was also extended to denote youthful immaturity, innocence, or naivety.
But it seems to me they're a thought too young and tender for the work in hand. It's bitter cold up at the Front now."
— Rudyard Kipling, "The Drum of the Fore and Aft," 1888
That childish sense led to such constructions as "tender age" and "tender years," which first appear as early as the 14th century in reference to children and young adults not strengthened by age, experience, or exercise.
… soothly in any treatise of the astrolabe that I have seen there be some conclusions that will not in all things perform their behests. And some of them be too hard to thy tender age of ten year to conceive.
— Geoffrey Chaucer, A Treatise of the Astrolabe, 1391There came two Springals of full tender years. / Farre thence from forrein land, where they did dwell….
— Edmund Spenser, The Faerie Queene, 1596
The sensitive bodily and emotional senses of tender dealing with the antagonistic forces of love and pain tip-toed into the English language in the 15th century, but they did not go unnoticed by William Shakespeare.
… I love Valentine, / Whose life's as tender to me as my soul.
— The Two Gentleman of Verona, circa 1590Look as the full-fed hound, or gorged hawk / Unapt for tender smell or speedy flight….
— The Rape of Lucrece, 1594In faith, I do not love thee with mine eyes, / … Nor are mine ears with thy tongue's tune delighted, / Nor tender feeling, to base touches prone…
— "Sonnet 141," 1609But, conscience, conscience,— / O 'tis a tender place, and I must leave her.
— King Henry VIII, 1613
Animals and plants also need TLC, but it wasn't until the early 17th century that farmers cultivated tender as an antonym of hardy, applying it to climate-sensitive plants—as well as animals (turkeys are considered "very tender to bring up")—needing protection and a lot of attention. It is also in the 17th century that tender developed use synonymous with touchy in reference to matters that needed to be approached delicately or cautiously so as to not offend.
… they both felt that the situation was extremely tender and critical….
— William Makepeace Thackeray, Vanity Fair, 1848
And the considerate people who effectively dealt with tender matters were said to be "tender" themselves.
Mrs. Smith did not want to take blame to herself, and was most tender of throwing any on her husband….
— Jane Austen, Persuasion, 1817
In 1954, the Broadway play Tender Trap, written by Max Schulman and Robert Paul Smith and later adapted as a movie starring Frank Sinatra and Debbie Reynolds, rekindled the "loving" sense of tender, and gave it a twist. The play centers around relationships and marriage, and its title came to refer to such romance, but it also sparked a figurative meaning referring to a seemingly attractive situation that has hidden danger (like in any romantic relationship). In the movie Sinatra sings:
You see a pair of laughing eyes, and suddenly you’re sighing sighs. You’re thinking nothing’s wrong, you string along, boy, then snap. Those eyes, those sighs, they’re part of the tender trap.
Other senses of tender relate to the French word denoting the act of offering (based on Latin tendere, "to stretch, hold forth"). Tender is the name of a small boat that offers assistance to a larger vessel, especially to transport passengers or goods to shore. It's also employed in the term legal tender to signify money that a creditor is obligated to accept in satisfaction of a debt. That sense may seem a bit odd today (after all, isn't all money "legal"?), but in the past when bartering was common (as well as the exchange of gold and silver), the introduction of official notes of currency became an issue.
A historical example goes back to the American government's financing of the Civil War, during which the government began issuing paper notes, popularly known as greenbacks under the Legal Tender Act of 1862, which declared them to be "legal tender" and thus compelled creditors to accept them as payment. Naturally, creditors who had lent gold or silver didn't want to be repaid in greenbacks, and debtors who borrowed paper didn't want to have to repay in precious metal if the law got struck down.
Another tender is someone who attends to or takes care of someone or something. That one has French roots but is believed to be an aphetic (shortened) form of attend and intend, from Anglo-French atendre and entendre. This tender was first used circa 1500 for someone who tends to the needs of or waits on another, but as a nurse or hostler—or someone who tends to the functioning of a machine. It wasn't until the 19th century that tender referred to waiters and bartenders.
In the end, remember that "tender" people and things (and Tenderheart Bear) might be soft, delicate, or sensitive, but they have great significance. People of a tender age are loved; legal tender is appreciated; any tender meat or vegetable is usually delectable; and tender moments (perhaps fueled by a bartender) are unforgettable.