Man walks into a bar and proceeds to get increasingly drunk over the
next couple of hours.  Suddenly he spots a guy in the corner with a
group of friends and begins to verbally abuse him.

"Oi you!!" he shouts "I've shagged your mum!!".
The man carries on drinking trying to ignore the guy's drunken rantings.

Half an hour later the man stands up and continues his abuse.

"Oi you!!!"  he shouts even louder this time "I shagged your mum up the arse!!"

The guy in the corner turns his back on him and continues talking with
his friends although by now visibly irate.

Half an hour later the guy pipes up once again:

"Oi you!!, your mum sucked my cock!!!!!"

By now the guy in the corner has lost his patience, he stands up furiously
and  retorts:

" Dad, shut the fuck up!"