Interesting and puzzling. The very first definition of 'rag' in the One-Look side bar is:

noun: a boisterous practical joke (especially by college students)

That's what I've always understood it to mean; the definition that y'all seem to know comes in as the 10th given in One-Look:

verb: censure severely or angrily

I'm confused 'cos I thought One-Look originated in the US of A. Perhaps there is some unexplained order in the way they cite the definitions?