late 14c., from Old French sorplus "remainder, extra" (12c., Modern French surplus), from Medieval Latin superplus "excess, surplus," from Latin super "over" (see super-) + plus "more" (see plus). As an adjective from late 14c.
word-forming element meaning "above, over, beyond," from Latin super (adverb and preposition) "above, over, on the top (of), beyond, besides, in addition to," from *(s)uper-, variant form of PIE root *uper "over." In English words from Old French, it appears as sur-. The primary sense seems to have shifted over time from usually meaning "beyond" to usually meaning "very much," which can be contradictory. E.g. supersexual, which is attested from 1895 as "transcending sexuality," from 1968 as "very sexual."
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.