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.