Shannanigans are similar to shenanigans, but in the case of shannanigans, they are perpetrated by Shan. It can be assumed that everything Shan does is shannanigans.
that restaurant with all that goofy shit on the walls and mozerella sticks in vermont that a certain officer (farva) enjoys
Captain O'Hagan: I swear to God I'm going to pistol whip the next guy who says, " shananigans."
Mac: Hey Farva what's the name of that restaurant you like with all the goofy shit on the walls and the mozzarella sticks?
Farva: You mean shananigans?
Mac: OOOOOOOOOOOOOO.
Thorny: OOOOOOOOOOOOOO.
as they hand the Captain their pistols
When you beleive that there is deception or foul play afoot, and you want to be the one that calls it.
You stand up, point at the object/person that you believe to have been involved in the deception and yell "Shannanigans!!" as loud as you can as to attract attention.
However there is an element of risk involved. If you are wrong about the "Shannanigans" call and there is nothing sinister about the situation, you will look like a douchebag
everybody had a hearty chuckle at Jerry when he called Shannanigans on Tom, thinking that he has slept with his wife. Only to find out that he was dead wrong.