Green’s Dictionary of Slang

mark n.2

In phrases

when the mark buss (also when the mark bust, ...burst) [Carib.E. mark, one of the 36 symbols – a centipede, a hog, an old lady – used in the gambling game of Whe-Whe. Each is identified by a number and players bet on which number will be found to reveal the winning symbol in a round]

(W.I.) when the truth comes out, when the facts are revealed.

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[WI]cited in Allsopp Dict. Carib. Eng. Usage (1996).