The Collaborative International Dictionary of English v.0.48:
Slack \Slack\, a. [Compar. Slacker; superl. Slackest.] [OE.
slak, AS. sleac; akin to OS. slak, OHG. slah, Prov. G.
schlack, Icel. slakr, Sw. slak; cf. Skr. s[.r]j to let loose,
to throw. Cf. Slake.]
Lax; not tense; not hard drawn; not firmly extended; as, a
2. Weak; not holding fast; as, a slack hand. --Milton.
3. Remiss; backward; not using due diligence or care; not
earnest or eager; as, slack in duty or service.
The Lord is not slack concerning his promise, as
some men count slackness. --2 Pet. iii.
4. Not violent, rapid, or pressing; slow; moderate; easy; as,
business is slack. "With slack pace." --Chaucer.
C?sar . . . about sunset, hoisting sail with a slack
southwest, at midnight was becalmed. --Milton.
Slack in stays (Naut.), slow in going about, as a ship.
Slack water, the time when the tide runs slowly, or the
water is at rest; or the interval between the flux and
reflux of the tide.
Slack-water navigation, navigation in a stream the depth of
which has been increased, and the current diminished, by a
dam or dams.
Syn: Loose; relaxed; weak; remiss; backward; abated;
diminished; inactive; slow; tardy; dull.