What is the X in X-mas?
1570s, the oral rendering of the arithmetical sign +, from Latin plus "more, in greater number, more often" (comparative of multus "much"), altered (by influence of minus) from *pleos, from PIE *pele- (1) "to fill" (see poly-).
As a preposition, between two numbers to indicate addition, from 1660s. [Barnhart writes that this sense "did not exist in Latin and probably originated in commercial language of the Middle Ages."] Placed after a whole number to indicate "and a little more," it is attested from 1902. As a conjunction, "and," it is American English colloquial, attested from 1968. As a noun meaning "an advantage" from 1791. Plus fours (1921) were four inches longer in the leg than standard knickerbockers, to produce an overhang, originally a style associated with golfers. The plus sign itself has been well-known since at least late 15c. and is perhaps an abbreviation of Latin et (see et cetera).