Origin and history of testes

"a testicle," 1704, from Latin testis (plural testes) "testicle," a word usually regarded as a special application of testis "witness" (see testament) on the notion of what "bears witness to male virility" [Barnhart]. Stories that trace the Latin word to some supposed swearing-in ceremony are groundless modern inventions.

Compare Greek parastatai "testicles," from parastates "one that stands by;" and French slang témoins, literally "witnesses." But Buck thinks Greek parastatai "testicles" has been wrongly associated with the legal sense of parastatēs "supporter, defender" and suggests instead parastatai in the sense of twin "supporting pillars, props of a mast," etc. Or it might be a euphemistic use of the word in the sense "comrades." OED (1989), meanwhile, points to Walde's suggestion of a connection between testis and testa "pot, shell, etc." (see tete).