Best definition
wanker flag
Flag depicting the cross of St. George (red cross on white background) carried by loud football fans, or attached

to their cars. So called because it gives the bystander a good clue as to the masturbatory proclivities of the bearer.
Mainly found in southern U.K.
Bearers of wanker flags can often be heard making noises similar to ‘ING-UH-LAAND’, ‘TOOO NIL’, and other such inanities.

I was going to go down the pub for a swift pint but it was full of twats carrying wanker flags!