noun

• (uncountable) The part of anything that hangs loose, having no strain upon it.
  "the slack of a rope or of a sail"
• (countable) A low-lying marsh or a pool, especially a tidal or intermittent one which periodically fills and drains.
  "The "slacks" I have mentioned are fresh-water pools which extend just inside the outer sandhills. Being mostly dry in summer, the shore fowl love to breed there. Peewits nest on their banks, and the long grasses and sand willow[…]"
• (uncountable, psychotherapy) Unconditional listening attention given by client to patient.
  "The counselor is directed to give his client "free attention," or "slack," performing a kind of vigil, a version of Carl Rogers's "unconditional positive regard.""

adj

• (normally said of a rope) Lax; not tense; not firmly extended.
  "a slack rope"
• (slang, Caribbean, Jamaica) Vulgar; sexually explicit, especially in dancehall music.
• (linguistics) Lax.

verb

• (ambitransitive) To slacken.
  "I maruell I heare no nevves of Dromio, either hee ſlackes the matter, or betraies his Maiſter, I dare not motion anie thing to Stellio, till I knovv vvhat my boy hath don, Ile hunt him out, if the loiterſacke be gone ſpringing into a Tauerne, Ile fetch him reeling out."
• (obsolete) To mitigate; to reduce the strength of.
  "Ne did ſhe let dull ſleepe once to relent, / Nor wearineſſe to ſlack her haſt, but fled / Euer alike,[…]."

noun

• (rail transport) A temporary speed restriction where track maintenance or engineering work is being carried out at a particular place.
  "This pitfall, beginning in February and finishing in May, resulted in a drop of about 3 ft. in the platform level; during this period it was necessary to level the track three times weekly, and impose a service slack of 15 m.p.h. The subsidence appears now to have finished, and normal speed is once again permitted."