a gent & his mates go to a mixed bar. A group of ladies flirt with them, but unfortunately one of the ladies is VERY ugly (for example, see whalefuck or sealfuck). She is "the grenade". Now, the chicks likely have a deal amongst themselves, if one isn't getting fucked, then they are all abstaining. It is the duty of one of the gents to "fall on the grenade" , that is, to fuck this dog of a coyote ugly chick
One Saturday night I fell on the grenade for my buddies and it turned out to be the hottest fuck I'd ever had. But I didn't want to ever be seen with her again.
Often one will go out to eat at McDonalds, only for the mexican dude at the cash register to mess your order up. A McAsshole is the stupid guys at McDonalds behind the counter constantly messing peoples simple orders up.
A: Dude all I asked for was a friggin McSalad shaker and that McAsshole gave me chicken McNuggets!
B: Lets McJump him after his shift is over.
A: Good idea. In the mean time I'll go take a McShit.