boil over
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.
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Source: A Dictionary of American Idioms