What does the term sacked mean?

The term sacked (literally to place in a sack or bag) has more than one idiomatic use.

Historical warfare

To "sack" a city was to plunder its contents. The city of Rome was famously sacked by the Visigoths in 410 AD, ending the Western Roman Empire, and also in 1527 AD by mutinous troops of the Papal States.

Employment

The term "sacked" means fired, or terminated.

US football

Applied to an attempted pass play in football, it means that the quarterback was tackled behind the line of scrimmage, sometimes for a significant loss of yardage. The term was coined by Los Angeles Rams defensive end David "Deacon" Jones, who likened it to sacking and pillaging a city.