it is when you are about to fart and you say to someone "knock knock" and they say "who's there?" and you say, "Bruce." and they say, "Bruce who?" and you say, "Bruce Wetness" and then let it go and fart...this joke only works if it is a wet fart. Then you say, "Get it? Bruce Wetness like Bruce Willis" CAUTION: prepare to need new underwear after this joke is administered.
"oh no I need to borrow your underwear because I just had a meeting with Bruce Wetness...he's a great actor"