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".