Английский Этимологический словарь - slack (adj.)

- O.E. slжc "loose, careless," from P.Gmc. *slakas. Sense of "not tight" is first recorded 13c. The noun meaning "loose part or end" is first recorded 1794; that of "quiet period, lull" is from 1851. Slacks "loose trousers" first recorded 1824, originally military. Slacker popularized 1994, though meaning "person who shirks work" dates back to 1898.