('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'.
(..) Please tell me it's daylight. Otherwise, I don't wanna know. Uh... No, it's not daylight. Oh, we're in deep schist. I wish there were seat belts on this thing. You're worried about seat belts? - Whoa! - Ah! Watch it. (..)2008 Journey to the Center of the Earth