Definitions of slack:

• noun:   a cord or rope or cable that is hanging loosely

  Example: "He took up the slack"

• noun:   the condition of being loose (not taut)

  Example: "He hadn't counted on the slackness of the rope"

• noun:   a stretch of water without current or movement

  Example: "Suddenly they were in slack water"

• noun:   dust consisting of a mixture of small coal fragments and coal dust and dirt that sifts out when coal is passed over a sieve
• noun:   a noticeable deterioration in performance or quality

  Example: "A gradual slack in output"

• verb:   cause to heat and crumble by treatment with water

  Example: "Slack lime"

• verb:   avoid responsibilities and work, be idle
• verb:   make less active or fast

  Example: "He slackened his pace as he got tired"

• verb:   release tension on

  Example: "Slack the rope"

• verb:   be inattentive to, or neglect

  Example: "He slacks his attention"

• verb:   make less active or intense
• verb:   become less in amount or intensity
• verb:   become slow or slower
• adjective:   flowing with little speed as e.g. at the turning of the tide

  Example: "Slack water"

• adjective:   not tense or taut

  Example: "Slack and wrinkled skin"

• adjective:   lacking in rigor or strictness

  Example: "Slack in maintaining discipline"

• adjective:   lacking in strength or firmness or resilience

  Example: "A slack grip"

• name:  A surname (rare: 1 in 25000 families; popularity rank in the U.S.: #3139)