To "frank" someone when in a social situation (i.e. a pub) involves dipping or placing a frankfurter into their drink. This is done either when they get up to go to the toilet/bar and leave their drink unattended, or when they are sat at the table before they realise what you are doing.
1. "Quick he's gone to the toilet let's frank his pint!"
2. He really got franked there when he left the table