slack adj.

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.

early 14c., "cessation" (of pain, grief, etc.), from slack (adj.).

The meaning "a cessation of flow in a current or tide" is from 1756; that of "still stretch of a river" is from 1825. The meaning "quiet period, lull, interval in activity," especially in reference to business, is by 1851.

The meaning "loose part or end" (of a rope, sail, etc.), having no stress upon it, is attested from 1794; hence the figurative senses in take up the slack (1930 figuratively) and slang cut (someone) some slack (1968).

"coal dust," mid-15c., sleck, a word of uncertain origin, probably related to Middle Dutch slacke, Middle Low German slecke "slag, small pieces left after coal is screened," which is perhaps related to slagge "splinter flying off metal when it is struck" (see slag (n.)).

1510s, transitive, "to moderate, make slack," a back-formation from slack (adj.) after the original verb in this sense, slake (q.v.), veered into a specialized sense.

The intransitive meaning "be slack; be remiss, inactive, or idle; fail to exert oneself" is attested from 1540s; current use in this sense is probably a re-coining from c. 1904 (see slacker, and compare Old English slacful "lazy," sleacmodnes "laziness"). Related: Slacked; slacking.