1570s, the oral rendering of the arithmetical sign +, also "more by a certain amount" (correlative to minus), from Latin plus "more, in greater number, more often" (comparative of multus "much"), altered (by influence of minus) from *pleos, from PIE root *pele- (1) "to fill" (see poly-). The plus sign itself has been well-known at least since late 15c. and is perhaps an abbreviation of Latin et (see et cetera).
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;" OED writes that "the words plus and minus were used by Leonardo of Pisa in 1202."] Placed after a whole number to indicate "and a little more," it is attested by 1902. As a conjunction, "and, and in addition," it is American English colloquial, attested by 1968. As a noun meaning "an advantage" from 1791. Plus fours "distinctive style of long, wide knickerbockers" (1921) were four inches longer in the leg than standard knickerbockers, to produce an overhang, originally a style associated with golfers.