Greebo refers to the Stourbridge music scene of the early 90s, championed by such bands like Ned's Atomic Dustbin.
For some reason, it's now attributed to a subculture who thinks they're above another sub-culture (the so called "chavs"), to the extent they wish all the "chavs" were dead. As you can tell, they're not that familiar with the words "irony" or "hypocrite".
Greebo: all you chavs are the same!
Chav: all you greebos are the same!
Everyone else: *rolls eyes*