Asides from being the truth it is an oblique way of expressing "All cops are bastards" since they share the same acronym.

A woman in Spain was arrested for a bag with ACAB written on it but was released when it was found it apparently stood for this above expression.

Always carry a Bible is a similar expression that is more often used as a cover for when a tattoo that was chosen without too much thought.
When I see a legal criminal walking down the street I let them see that my T-shirt says : All Cats Are Beautiful.
by GiraffeDoor June 11, 2018
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