A phrase in place of 'give it up' or 'forget about it'. It has even been used in place of 'can't be bothered'.
Q-Do you want to do some work today?
A-Nah, sack it off.
Q-They've already gone. If we left now, we could probably catch them up...?
A-Nah, sack it.
to deliberately not do something/ to avoid something/ the act of ditching a person or thing.
variations: sack it/him/her off, sacking off.
A: wats good? wanna come cotch at mine?
B: cant, got an essay to do and some reading aswell, long.
A: sack it off - we'll smoke a joint and play pro instead.
A: Alex knew we were leaving at 7 where the fuck is he?
B: Allow it, sack him off we're going.
I've been a wasteman, sacking off all day.
A: you do your homework?
B: allow it, i sacked it off.