Three young lads went on there usual rounds around the pubs. They came to one pub got themselves a drink and after 5 minutes a old drunken bloke in the corner staggers over to them and says, "I've kissed your mom".
One of the lads turns to the drunk and gives him the eye and turns back to his drink, the drunk also returns.
The drunk returns another 5 minutes later and shouts, "I've licked your mom out."
The same lad turns around bites his lip and returns to his drink with his friends, the old drunk also returns.
As you've guessed it the old drunk staggers over to them again and says, "I've had an affair with your mom for the last 20 years."
The same lad as last time stands up and says "go home dad!"