A few minutes ago I said ‘don’t get shirty!’ to my husband in a joking manner, and we had a bit of a laugh and my daughter then wondered where shirty came from. I must confess I don’t know. It means of course don’t get cross or annoyed, and I would think it’s probably quite a mild thing, and possibly quite an old fashioned thing to say. I would also guess that the person saying it might also be feeling a little shirty! It must be very closely connected to ‘keep your shirt on!’ which means much the same thing. Does it originate from people pulling off their shirts to have a fight?
Apparently, according to some etymological sites it comes from ‘getting your shirt out’, which I suppose is untucking it from your trousers (or skirt if you happen to be wearing one, or even a kilt!) It may date back to the 1840’s, but that only means that was when it was noted in a dictionary or article about words, expressions and their meaning. I also learned that it can be used the other way round, meaning to annoy or aggravate someone – to get their shirt out, as in ‘I really got his shirt out!’
There are a few other clothing/anger expressions – getting hot under the collar, flipping your wig, losing your rag, and my favourite, getting your knickers in a twist. My favourite etymological site, has a rather lovely example: ill-tempered,” 1846, slang, probably from shirt (n.) + -y – on notion of being disheveled in anger. Dishevelled in anger, I like it!!