('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'.
(..) Are you crazy? My parents are here. That's weird, I... don't see them. Was this your plan all along? To... to get in my room and then... - get another notch on your belt. - No. Nancy, no. I'm not Laurie, or Amy, or Becky. You mean, you're not a slut. That's not what I'm saying. (..)2016 TvShow: Stranger Things Title: Chapter One: The Vanishing of Will Byers Season: 1 Episode 1