A.C.A.B. stands for ‘All Cop[per]s Are Bastards.’ Dating from at least the 1940s, and used as a slogan during the UK miners’ strike, A.C.A.B. is an abbreviation often integrated into prison tattoos in the United Kingdom, it is most commonly rendered with one letter between the knuckle and first joint of each finger, alternatively sometimes seen as symbolic small dots across each knuckle.
The British Oi! punk band, the 4-Skins, popularized the acronym in their 1980s song of the same name. It is currently in common usage as a phrase and tattoo amongst some ultras ( a type of sports fans renowned for their fanatical and elaborate displays of ‘support’) groups in Europe, particularly in Eastern Europe where most groups are more akin to hooligans than ultras.