A mussel party is a party attended by a bunch of crunchy old people. During this party they consume mollusks and drink cheap liquor. This is the old people version of a kegger, only instead of leading to vomiting it leads to anal explosions.
Me: Hey mom and dad how was the mussel party?
Mom: It was great! We ate a lot of mussels and drank some good wine.
Dad: It was terrible... I just turned the bathroom into a terrorist hot spot.
Me: That sounds like a terrible time.
Heinous sexual act. Sub folds limbs into a pretzel shape, Dom cuts a lemon in half and squeezes juice into Sub's asshole, then leaves the two halves of the lemon on top of the nipples of Sub while sodomizing him/her.
Girl 1: Hey, I heard you and Derek are into crazy sex.
Girl 2: Yeah, it's ridiculous. Last night we did this thing called the mushy lemon pretzel. It's insane.
Girl 1: Oh? I've never heard of that one. Sounds tangy