('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'.
(..) We're going in. I'm taking Neo to see her. See who? The Oracle. Everyone please observe ... ... the "fasten seat belt" and "no smoking" signs are on. Sit back and enjoy your flight. We're in. We'll be back in an hour. Unbelievable ... (..)1999 The Matrix
(..) part of a trace program. It disrupts your carrier signals so we can pinpoint your location. What does that mean? It means buckle your seat belt, Dorothy ... ... because Kansas ... ... is going bye-bye. Did you ... ? Have you ever had a dream, Neo, that you were so sure was real? (..)1999 The Matrix