From an episode of Family Guy where Peter tries to "educate" himself on proper edicate and ends up picking up a New Yorker. Being Peter, he turns to the comic section of the New Yorker and find "I would be more apathetic if I weren't so lethargic." And waits a few days trying to understand the meaning of it.
>"I would like a copy of the New Yorker Please"
>>"Okay"
>"I would be more apathetic if I weren't so lethargic? What?"
>"Ohhh I get it, can I get a jugs?"
>> "Sure thing"
"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...