quadruplet n.

"consisting of four parts, fourfold, four times told," 1550s, from French quadruple (13c.), from Latin quadruplus "fourfold," from quadri- "four" (from PIE root *kwetwer- "four") + -plus "-fold" (see -plus). Earlier in English as a noun, "a fourfold amount," early 15c., from Latin quadruplum.

1650s, "three successive lines of poetry," from triple; perhaps patterned on couplet. Extended to a set of three of anything by 1733, and to three children at the same birth by 1787 (another word for this was trin, 1831, on the model of twin). Musical meaning "three notes played in the time of two" is from 1801.

1820 as a shortening of quadrangle (n.) in the building-court sense (in this case "quadrangle of a college," Oxford student slang); also in old slang the quadrangle of a jail or prison, where prisoners take their exercise. By 1880 as short for the printer's quadrat (n.). By 1896 as "a quadricycle, a bicycle for four riders" (quadricycle is attested by 1879, quadruplet in this sense by 1893). As "one of four young at a single birth" by 1951 (in reference to armadillos), short for quadruplet; 1970 as a shortening of quadraphonic (adj.). Related: Quads.