The antonym for the phrase "chilled off" is heated up. When something is chilled off, it means that it has cooled down or lost its heat. On the other hand, when something is heated up, it means that it is becoming hotter or increasing in temperature. Therefore, these two terms are opposites of each other.
For instance, when cooking a stew, you might have to chill off the excess heat before adding in more ingredients. An antonym for chilling off in this context would be to heat up the stew again. Similarly, when sitting in a cold room, you might need to put on a sweater to warm up. The antonym for this scenario would be to take off a layer of clothing to chill off.