sextuplet n.

"sixfold, six times as much," 1620s, ultimately from Latin sextus "sixth" (from sex "six;" see six) + -plus "more; -fold" (see -plus). Compare French sextuple, Spanish sextuplo, Italian sestuplo. As a verb, "multiply by six," from 1630s.

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.