Slang for "get rid of".
It is often used in the context of a getting fired from a job.
It is also used in the context of someone getting punched or kicked very hard in the testicles, probably because the person doing the sacking is "getting rid of" the ability to have children in the person being sacked.
"John got sacked yesterday, and now he doesn't have a stream of income."
"John got sacked yesterday by me and the boys, and now he probably can't have children."