slackly (adv.)

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.

common adverbial suffix, forming from adjectives adverbs signifying "in a manner denoted by" the adjective, Middle English, from Old English -lice, from Proto-Germanic *-liko- (cognates: Old Frisian -like, Old Saxon -liko, Dutch -lijk, Old High German -licho, German -lich, Old Norse -liga, Gothic -leiko); see -ly (1). Cognate with lich, and identical with like (adj.).

Weekley notes as "curious" that Germanic uses a word essentially meaning "body" for the adverbial formation, while Romanic uses one meaning "mind" (as in French constamment from Latin constanti mente). The modern English form emerged in late Middle English, probably from influence of Old Norse -liga.