Green’s Dictionary of Slang

1. an aggressive phr. used in London streets, usu. greeted with the equally aggressive rejoinder ‘Who are you?’.

1837		‘Who Milked My Cow?’ in Bentley’s Misc. Jan. 88: As I passed down Arlington-street, a fellow stared at me and shouted ‘Who are you?’.
1841		C. Mackay Memoirs of Extraordinary Popular Delusions (1869) 244: When this phrase had numbered its appointed days, it died away like its predecessors, and ‘who are you?’ reigned in its stead. This new favourite, like a mushroom, seems to have sprung up in a night, or, like a frog in Cheapside, to have come down in a sudden shower. One day it was unheard, unknown, uninvented; the next it pervaded London. Every alley resounded with it; every highway was musical with it [...] The phrase was uttered quickly, and with a sharp sound upon the first and last words, leaving the middle one little more than an aspiration. Like all its compeers which had been extensively popular, it was applicable to almost every variety of circumstance.
1909		J. Ware Passing Eng. of the Victorian Era 265/1: Who are yer? (Street, 1883). Enquiry in an offensive tone, made in the street, and which, when answered, usually receives the counter-enquiry, ‘who are you?’.
1926		C. Ryan ‘From “Quoz” to “Razzberries”’ in AS II:2 91: ‘Who are you?’ supplanted the longer expression, ‘Does your mother know you’re out?’.

2. (US campus) what’s the matter, what’s your problem?

1989

P. Munro Sl. U.

3. (US campus) as excl., be quiet!

1989

P. Munro Sl. U.