The Collaborative International Dictionary
on the rag \on` the rag"\, pred. a. Menstruating; -- of women. [vulgar slang]
Wiktionary
a. 1 (context idiomatic vulgar slang euphemistic English) menstruate. 2 (context idiomatic English) In a bad mood.
Usage examples of "on the rag".
Deymorin's hand clenched on the rag, and Nikki expected any moment to see it fly across the room.
In her hurry she slipped on the rag rug and fell to the floor with a jolt but leaped up so quickly she was not even aware of the pain.
Sometimes it was a monthly, if Dave got sidetracked by other interests (maple-sugaring, cider-making, rocket-building, and car-customizing, just to name a few), and then there would be jokes I didn't understand about how Dave's Rag was a little late this month or how we shouldn't bother Dave, because he was down in the basement, on the Rag.
The Assistant Commissioner, with his eyes lowered on the rag of blue cloth, waited for more information.
A few lumps had fallen across the hearth, whilst others lay flaked and scattered on the rag mat beside the bed.
She swung her feet out of bed, and splashed them into the pool of sunshine rippling on the rag rug.