(uncountable) The part of anything that hangs loose, having no strain upon it. / (countable) A tidal marsh or shallow that periodically fills and drains.

(normally said of a rope) Lax; not tense; not firmly extended. / Weak; not holding fast. / Moderately warm. / Moderate in speed. / Lacking diligence or care; not earnest or eager.

Slackly.

To slacken. / (obsolete) To mitigate; to reduce the strength of. / To lose cohesion or solidity by a chemical combination with water; to slake.

(rail transport) A temporary speed restriction where track maintenance or engineering work is being carried out at a particular place.

(countable) A valley, or small, shallow dell.

(mining) Small coal; coal dust.

The state of being slack; the quality of having slack. / Vulgarity in West Indian culture. / A subgenre of dancehall music with sexual lyrics.

plural of slack

(dated) Semi-formal trousers which are less formal than those that are part of a suit but nowadays are considered suitable casualwear in most offices. (Takes a plural verb even when referring to a single pair; may be referred to as a pair of slacks.)