plural (adj.)

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.

1818, as a term in church administration, "the holding by one person of two or more offices at the same time," from plural + -ism. Attested from 1882 as a term in philosophy for a theory which recognizes more than one ultimate principle. In political science, attested from 1919 (in Harold J. Laski) in the sense of "theory which opposes monolithic state power." General sense of "toleration of diversity within a society or state" is from 1933. Related: Pluralist (1620s, in the church sense); pluralistic.