To lose one’s rag is to lose one’s temper and seems to be a modern expression i.e. first of the 20th century. It is used on both sides of the Atlantic but the origin remains obscure. The best explanation, but unverified, is that rag is slang for tongue and when one loses one’s rag, one loses control of one’s tongue. The major problem with this, however, is that rag or red rag meaning tongue dates back to the 16th century. Its sudden re-appearance in this sense some four hundred years later, in etymological terms, is very dubious to say the least. Another origin, sometimes encountered, that it refers to menstruation is not only distasteful but also completely unfounded.