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1. To increase in heat; to heat up (something). Primarily heard in UK. I'm just waiting for this stew to hot up, and then we'll be ready to eat! I'm sorry about that, sir, would you like me to hot up that steak for you?
2. To make or become more intense, interesting, or exciting. Primarily heard in UK. The election campaigns for the local MP are really hotting up. I feel like my relationship with Darren is really beginning to hot up.