When you fuck a girl in a field of mulch (near a farm) and you shove a small garden shovel up a girls ass and pour lemon juice down the hole. Then the guy has to drink everything that comes out of the hole and the girl has to take a shit.
I gave the fat chick a lemon limefarmer's surprise after i found out she liked shovels in her ass.
"That's so lemon" is a way of saying something is very cool but slightly off, in the way that eating a lemon or something of that highly acidic or citrusy nature causes one to make /that/ face.
A: That new mansion I bought is so cool!
B: Yeah, that's so lemon! Too bad it's decrepit and beyond repair...