slouch n.

Middle English slak, of persons, "indolent, lazy;" also (from c. 1300), of things or parts, "loose, not tight or taut;" from Old English slæc "remiss, lax, characterized by lack of energy, sluggish, indolent, languid; slow in movement, gentle, easy," from Proto-Germanic *slakas (source also of Old Saxon slak, Old Norse slakr, Old High German slah "slack," Middle Dutch lac "fault, lack"), from PIE root *sleg- "be slack, be languid" (languid is an IE cognate of it).

As an adverb from late 14c. Slack-key in reference to guitar tunings with looser strings (1975) translates Hawaiian ki ho'alu. Slack water (n.) is from 1769 as "time when tide (high or low) is not flowing" (slake-water is from 1570s); as "part of a river behind a dam" by 1836, especially American English.

Formerly common in depreciative compounds such as slack-jawed (q.v.), slack-handed "remiss, negligent" (1670s). Slack-baked "baked imperfectly, half-baked" is from 1823; used figuratively from 1840. The 17c. had slack-hammed. Slack and slow was a Middle English alliterative pairing.

late 12c., slouthe, "indolence, sluggishness, neglect of responsibilities," formed from Middle English slou, slowe (see slow (adj.)) + abstract formative -th (2).

It displaced earlier sleuthe, from Old English slæwþ, Kentish slewð, "sloth, indolence." The modern form might also be the old word conformed to the vowel in the adjective. The modern sense of "slowness, tardiness" is attested from mid-14c. As one of the deadly sins, it translates Latin accidia. A sloth-salve (c. 1400) was a (figurative) remedy for indolence.

The slow-moving South American mammal was first so called 1610s, a translation of Portuguese preguiça "slowness, slothfulness," from Latin pigritia "laziness" (compare Spanish perezosa "slothful," also "the sloth").