When the 'lads' are in the pub/bar, one of them goes to the toilet and leaves their pint on the table. Another guy picks up the 'absentees' pint and rubs the rim of the glass in between his foreskin and bell-end, when finished replaces the pint where it laid originally. You lie in wait for the victim to return, and then piss your selves laughing when he remark how shit the beer is!! Brilliant joke until it happens to you!
'I became a victim of rimming last night, that cunt Dan rimmed my beer!'