Green’s Dictionary of Slang

scalp v.3

[SE scalp, to remove the enemy’s scalp as a trophy]

(N.Z.) to capture the insignia or patch n. (3) of a rival gang-member.

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Sun. Star-Times (NZ) 3 Feb. A2: Detective Constable Brett Watene, the Tokoroa police iwi liaison officer, said beating and taking the patch of a rival gang member is known as scalping – and regarded as a declaration of war [DNZE].