When you use a public restroom (e.g. at work) and are about to have a loud and embarrassing session, but another person comes in, causing you to hold it until they leave. After they leave you are free to let loose without fear of being laughed at.
Popular Brit vernacular used when you just can't face taking part in something. First used in days of yore when people sat at the sides to miss a particular dance. Now it's used to skip meetings, when you're too drunk to dance with an ugly woman, or maybe skipping out on a party you know will suck weapon's grade ass.
"Dave, are you coming to Mike's birthday tonight?"
"Sorry mate, but he's a friendless loser, and I shagged his girlfriend. I think I'll Sit this one out, if that's alright with you".
"No worries fella."
The ultimate form of revenge that a hammered dude can pull on his friends. All that needs to be done is saying, "That's it (pause) the dick's comin out." followed by the pulling out of his penis. A last resort type of revenge that cannot be topped, at least in the mind of the influenced.
(friend who has johns keys) Hey john did you lose your keys?
(John) ...don't you f**k with me. That's it... the dick's comin out.(pulls out penis)