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.