('chiefly US') cities in the 'Rust Belt' [=an area formerly known for industry and manufacturing]
When you're riding in a car, you should always 'belt up' [=('US') 'buckle up'] for safety.
That remark was 'below the belt'. = That remark really 'hit' (him) 'below the belt'.
(..) No. No. - It's gonna happen, Edward. I've seen it. - It doesn't have to be that way. She's still hemorrhaging. Alice, make me a tourniquet. Your belt. Carlisle, what's my other option? Tie it above my hands. Carlisle! Go. - We could try to suck the venom out. (..)2008 Twilight