slacken v.

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.

word-forming element making verbs (such as darken, weaken) from adjectives or nouns, from Old English -nian, from Proto-Germanic *-inojan (also source of Old Norse -na), from PIE adjectival suffix *-no-. Most active in Middle English and early modern English, hence most verbs in -en are comparatively recent.

Middle English slaken, from late Old English sleacian, slacian "become slack or remiss; relax an effort" (intransitive); "delay, retard" (transitive), from slæc "lax" (see slack (adj.), and compare Middle Dutch, Middle Low German slaken).

The transitive sense of "make slack, loosen" (ropes, a bridle, etc.) is from late 12c. The sense of "allay, diminish in force or intensity, quench, extinguish" is from late 13c. in reference to fire, c. 1300 in reference to thirst, hunger, desire, wrath, lust, etc. The notion is "make slack or inactive." Related: Slaked; slaking.