What is the X in X-mas?
c.1600, "fleshy mass at the back of the testicles," Modern Latin, literally "that which is on the testicles," from Greek epididymis, a word probably coined by Greek anatomist Herophilus (c.300 B.C.E.) from epi "on" (see epi-) + didymos "testicle," literally "double, twofold" (adj.). "To save his Epididamies" [Richard Brome, "The Court Beggar," 1652].
epididymis ep·i·did·y·mis (ěp'ĭ-dĭd'ə-mĭs)
n. pl. ep·i·di·dy·mi·des (-mĭ-dēz')
A long, narrow, convoluted tube in the spermatic duct system that lies on the posterior aspect of each testicle and connects with the vas deferens.