1. Putting yourself or someone else on a list for an activity, class, etc.
2. When someone is instant messaging you and they suddenly sign off because of something you say (similar to "hang up" on the phone)
guy1: i heard this year mr. herbert is teaching woodshop.
guy2: i have to go sign up for that class!
girl1: what happened last night? you totally signed up on me!
guy3: sorry, my internet connection is horrible.
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?!?