"courage," literally "testicles, balls," 1932, in Hemingway ("Death in the Afternoon," an account of Spanish bull-fighting), from Spanish cojon "testicle," from Latin coleus "the testicles" (source of Italian coglione), literally "strainer bag," a variant of culleus "a leather sack," cognate with Greek koleos "sheath of a sword, scabbard." Both are said in some sources to be from PIE root *kel- (1) "to cover, conceal, save," but de Vaan finds it "Probably a loanword from a non-IE language, independently into Latin and Greek."
English had it as cullion a 16c. term of contempt for a man, "a mean wretch" (Shakespeare) also "a testicle" (Chaucer), from Middle English coujon, coilon (late 14c.), from Old French coillon "testicle; worthless fellow, dolt," from Latin coleus.