When he didn't get the pay raise he was expecting, he had to take another job to 'pick up the slack'. [=to make up for the money he was not making]
The manager has to 'take up the slack' when employees don't do their jobs correctly.
We left some 'slack' in the budget so that we can hire more people if we have to.
(..) for that pickaninny, ain't got no use for 'em, ain't that right, Doc? – You heard 'em! CANDIE: I'm gonna have to excuse Mr. Stonesipher's slack jawed gaze, he… he ain't never seen a nigger like you ever in his life. Ain't that right, Mr. Stonesipher? – That right. CALVIN: That matter… (..)2012 Django Unchained