Green’s Dictionary of Slang

Johnny Tapp v.

[rhy. sl. = crap v.2 (1)]

(Aus. prison) to defecate.

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[Aus]Tupper & Wortley Aus. Prison Sl. Gloss. 🌐 Johnny Tapp. To deficate [sic]. Rhyming slang for crap. Johnny Tapp is a well known NSW racing commentator.