A word that is almost exclusively used in Northern Ireland.
It is a rather simple insult/comeback, it means 'suck the back of them' with 'them' being testicles.
For non-Northern Irish people it is pronounced 'sucka-backy-im'. That's IM as in TIM.
John: Here mate that's my football, giz it back ya wank
James: Na mate my ma got me this one
John: Mate, its the same ball, just giz it back!
James, Na suckabackyem ye ain't gettin it.
You aint tellin' me that my neighbour is comin' too? Which mottafukka ordered this sucksack to join my party? Fuck him, he aint comin' in; this geek's a weirdo!