pons asinorum

noun

pons asi·​no·​rum ˈpänz-ˌa-sə-ˈnȯr-əm 

: a critical test of ability or understanding

also : stumbling block

Word History

Etymology

New Latin, literally, asses' bridge, name applied to the proposition that the base angles of an isosceles triangle are equal

First Known Use

1645, in the meaning defined above