Green’s Dictionary of Slang

mask n.

1. (N.Z. prison) a full facial tattoo.

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[NZ]D. Looser Boobslang [U. Canterbury D.Phil. thesis] 114/1: mask n. 1 a full facial tattoo; a fully tattooed face.

2. (N.Z. prison) a very unattractive person.

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[NZ]D. Looser Boobslang [U. Canterbury D.Phil. thesis] 114/1: mask n. 2 an ugly person [from the suggestion of his fellow inmates that he should ‘get a mask’].

In phrases

(v.)

(N.Z. prison) to erect an emotional wall.

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[NZ]D. Looser Boobslang [U. Canterbury D.Phil. thesis] 114/1: mask n. mask up (largely amongst male prison inmates) to consciously adopt a defensive emotional wall that provides a barrier between the man’s real feelings and the outward facade he presents to the inmate group.