('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'.
(..) Family's running. The drive out was pretty fun. What was your favorite part? Spitting out of the car window. - Definitely not when Dad was singing. - Wearing the sit belt. Oh. What about the time with the dinosaur. - Oh, that's the one. - That was great. Say cheese. (..)2015 Inside Out