Losing your rag?

I know this in the UK is used when we lose your temper. In the US it means something completely different
Does anyone know the origin of the phrase. Thanks

"The Dictionary of Contemporary Slang" by Tony Thorne" says this expression is of "obscure origin" but rag "...has meant variously one's tongue, a flag, to tease, to bluster or rage, but none of these senses can be definitively linked to the modern phrase." (Pantheon Books, New York, 1990).