Collocations for slack - adj.

be, feel, look, seem
become, fall, go
My jaw fell slack in disbelief.
Let the reins go slack.

leave sth
Leave the thread slightly slack to allow for movement.

very, fairly
completely
Some of the ropes were completely slack.

a little, slightly, etc.

be, seem
become
remain
The antiques business remained slack.

extremely, fairly, very, etc.
This season has been pretty slack for local hotels so far.

be
become, get

very
a bit, rather

in
She knew she had been very slack in her church attendance recently.

Slack is used with these nouns:
business, jaw, mouth