('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'.
(..) Can I be the first to say, "Ew!" We gotta go tell Gru! All right, I'm going in. If it picks up any traces of the serum, the sensor in your belt buckle will make a sound like this... Mee-maw! Mee-maw! Okay! I get it. I get it. Welcome to Eagle Hair Club. (..)2013 Despicable Me 2