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.

*pelə-, Proto-Indo-European root meaning "to fill," with derivatives referring to abundance and multitude.

It forms all or part of: accomplish; complete; compliment; comply; depletion; expletive; fele; fill; folk; full (adj.); gefilte fish; hoi polloi; implement; manipulation; nonplus; plebe; plebeian; plebiscite; pleiotropy; Pleistocene; plenary; plenitude; plenty; plenum; plenipotentiary; pleo-; pleonasm; plethora; Pliocene; pluperfect; plural; pluri-; plus; Pollux; poly-; polyamorous; polyandrous; polyclinic; polydactyl; polydipsia; Polydorus; polyethylene; polyglot; polygon; polygraph; polygyny; polyhedron; polyhistor; polymath; polymer; polymorphous; Polynesia; polyp; Polyphemus; polyphony; polysemy; polysyllabic; polytheism; replenish; replete; supply; surplus; volkslied.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit purvi "much," prayah "mostly;" Avestan perena-, Old Persian paru "much;" Greek polys "much, many," plethos "people, multitude, great number," ploutos "wealth;" Latin plus "more," plenus "full;" Lithuanian pilus "full, abundant;" Old Church Slavonic plunu; Gothic filu "much," Old Norse fjöl-, Old English fela, feola "much, many;" Old English folgian; Old Irish lan, Welsh llawn "full;" Old Irish il, Welsh elu "much."