palmaris brevis

noun

palmaris brev·​is -ˈbrev-əs 

: a short transverse superficial muscle of the ulnar side of the palm of the hand that arises from the flexor retinaculum and palmar aponeurosis, inserts into the skin on the ulnar edge of the palm, and functions to tense and stabilize the palm (as in making a fist or catching a ball)