('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'.
(..) in their cells, instinct that we can program. Their loyalty cannot be bought. These guys are gonna run straight into the enemy's teeth and eat them... belt buckle and all. What if they decide they wanna be in control? Then we remind them who is. We terminate the rogues. (..)2015 Jurassic World