O.E. slæc "loose, careless" (in ref. to personal conduct), from P.Gmc. *slakas (cf. O.S. slak, O.N. slakr, O.H.G. slah "slack," M.Du. lac "fault, lack"), from PIE base *(s)leg- "to be slack" (see lax
). Sense of "not tight" (in ref. to things) is first recorded c.1300.
The verb is attested from 1520; slacken (v.) first recorded 1580. Slack-key (1975) translates Hawaiian ki ho'alu First record of slack-jawed (1901) is in Kipling. Slack water "time when tide is not flowing" is from 1769. Slacker popularized 1994, though meaning "person who shirks work" dates back to 1898.