Lose your rag
Definition 1 of 1
Pro Tip 1/3Emotional Intensity
Indicates a sudden, intense burst of anger, often used when someone cannot control their feelings anymore. 
He was calm until the argument got personal, then he lost his rag.
Pro Tip 2/3Colloquial Usage
Primarily used in casual conversation, not in formal writing or professional environments. 
She's usually professional, but she lost her rag when the meeting ran over time.
Pro Tip 3/3British Expression
Recognize that it's a British phrase, may not be understood or used by English speakers from other regions. 
After living in London for a year, I've noticed people often say 'lose your rag' instead of 'get angry'.