When a group of friends find a dead bird (famously a grouse) the day after the night before whilst still relatively intoxicated and decide t'would be a good idea to use it to cause mischief and/or shennanigans. This could involve a good slap or as it should be known a grousin' to an unsuspecting pedestrians face! Alternatively you could dump it in a public urinal or in your flatmates frying pan. It helps if you drive a soft top car with the roof down playing the greatest hits of Justin Bieber and Baby Alice.
Billy Joel: "Shit dude, me and the guys got totally wasted last night and this morning we went for a bit of grousin' like you do."
Steve: "Grousin', what's that?"
Billy Joel: "You know, when you find a dead bird by the side of the road and use it to cause all sorts of crazy mischief!"
Steve: "Dude wtf is wrong with you? I hope you get caught"
Billy Joel: "I really don't know man, I hope we do too otherwise we'll never learn!"