Green’s Dictionary of Slang

rubber duck n.

[rhy. sl.]

1. sexual intercourse [= fuck n. (1a)].

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[Aus]Tupper & Wortley Aus. Prison Sl. Gloss. 🌐 Rubber duck. Rhyming slang for [...] fuck. Also a general term of abuse.
D. Shaw ‘Dead Beard’ at www.asstr.org 🌐 What the hell, I’m on holiday and getting a free rubber duck, so that can’t be mum and dad, can it?

2. (Aus. prison) fellatio [= suck n.3 (2)].

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[Aus]Tupper & Wortley Aus. Prison Sl. Gloss. 🌐 Rubber duck. Rhyming slang for suck (a reference to fellating).