('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'.
(..) They crawled. Their hands were bound. Their bonds were cut. They ran over here. They were followed. The belt! Run! Tracks lead away from the battle... ...into Fangorn Forest. Fangorn? What madness drove them in there? (..)2002 The Lord of the Rings: The Two Towers