What is the X in X-mas?
1590s, alteration of testicule (early 15c.), from Latin testiculus, diminutive of testis "testicle" (see testis). The Old English was herþan, probably originally "leather bag" (cf. heorþa "deer-skin"). The commonest slang terms for them in other languages are words that mean "balls," "stones," "nuts," "eggs."
testicle tes·ti·cle (těs'tĭ-kəl)
A testis, especially one contained within the scrotum.