What is the X in X-mas?
1888, from fellatio + -ion.
1887, from Latin fellatus, past participle of fellare "to suck," from PIE root *dhe(i)- (see fecund). The sexual partner performing fellatio is a fellator; if female, a fellatrice or fellatrix.
fellatio fel·la·ti·o (fə-lā'shē-ō', fě-)n. Oral stimulation of the penis.