Basically an even cooler way to say "cool." Can also refer to a place, be a conversation urger, or something to say when you have no idea what the other person is talking about. May have originated around the '50s, brought back by the movie Ghost World.
girl1: 8 mile is such an awesome movie!
boy1: yeah, coolsville.
boy2: Wanna go to the bowling alley tonight?
girl2: that place is coolsville!
teacher: we're going take the heterozygus allel for constricted pods and the homozygous allel for white seed color. aren't genetics fun?!?