"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...
B: Yeah, that's so lemon! Too bad it's decrepit and beyond repair...
by Loki May 1, 2005