means To warm up food or another substance that has already been cooked or heated once, so it's hot to eat or use again.

from A transparent compound of the prefix 're-, 'again' (from Latin re-), bolted onto 'heat,' which descends from Old English hǣtu, the warmth itself, sibling to 'hot.' So the word does exactly what it says on the tin: heat, then heat once more — English at its most plain-spoken.