slack-jawed 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.

late 14c., jowe, joue, "the bones of the mouth," "A word of difficult etymology" [OED]. Probably from Old French joue "cheek," originally jode, from Gallo-Romance *gauta or directly from Gaulish *gabata, but there are phonetic problems; or perhaps a variant of Germanic words related to chew (v.); compare also the two nouns jowl. Replaced Old English ceace, ceafl. Jaws as "holding and gripping part of an appliance" is from mid-15c.; figuratively, of time, death, defeat, etc., from 1560s.