(adjective) When a woman is "on the rag", that means she has her period. She's also likely in a bad mood because of the various discomforts associated with pads, leaks, tampons, washing out underwear, not being able to wear light-coloured skirts or pants, etc.
She's really in a mood, she's on the rag and complaining about everything. Last night she slept so soundly that she woke up in a pool of blood.
Its a type of slang word and basically means to pull someone by the hair or clothing very quickly. People do it when in a physical fight.
The Welsh use it alot.
Person 1: "My class is so boring. It's filled with a bunch of rags and a couple white kids who don't stop talking about Maplestory and Runescape."
Person 2: "I feel you, dude. That's the worst."
in older days, ladies used washable cloths (rectangular pieces) in the gusset of their underwear during menstruation or to prevent unforeseen pee discharge, the term was made synonymous to a menstruating lady having moods & quirks .