suckabackyem

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.