Board Thread:Fun and Games/@comment-27996636-20180225211948/@comment-32233739-20180305153242

lose his rag might be a bit of a british saying... it may even be more of a local manunian saying (Manchester) - like cockneys (londoners) have rhyming slang)

basically losing his rag means losing his temper, losing his S&!%, basically becoming very angry in reaction to something

for reference: https://en.wiktionary.org/wiki/lose_one%27s_rag