This is a nickname that uses reverse psychology on the classic word "Slacker" -as in to be lazy.
"Slack" is the most AWESOME word you can use to describe a person and has been developed to change expectation by subterfuge.
adult meanings: slacking, not working/ being active etc
Example: teenage mother: what ya doing with that in the house, setting bad examples for little timmy here!
teenage father: gimme some slack lauren, its not that illegal Timy: me want dope