('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'.
(..) And bring my vinyl bag marked "presidential palace." And hurry! Xania is in grave danger. Ponton, we must get to the presidential palace immediately! - Put your seat belt on. - I don't need a seat belt. Let's see what this baby can do! No. Clouseau should not be on this list. (..)1976 The Pink Panther Strikes Again