To heat up is to become warmer, or to become more intense. Heat up the soup, the argument heated up.
The figurative sense follows the literal exactly, since both describe rising energy. A dispute that begins to heat up is usually about something other than what is being said.
1phraseheat upTo become warmer, or to become more intense.
“The debate began to heat up about an hour in.”
heat up is a phrase.
Practised in Phrasal Verbs