boil over {v. phr.} 1. To rise due to boiling and overflow down the sides of a pan or a pot. * /"Watch out!" Jane cried. "The milk is boiling over on the stove!"/ 2. To become enraged to the point of being unable to contain oneself. * /John took a lot of abuse from his boss, but after 25 minutes he suddenly boiled over and told him what he thought of him./