Green’s Dictionary of Slang

chocka-block v.

also chocka
[? rhy. sl. = (get a mouthful of) cock n.3 (1)]

(Aus. prison) to fellate.

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[Aus]Tupper & Wortley Aus. Prison Sl. Gloss. 🌐 Chocka-block. To fellate. Probable rhyming slang with cock. Sometimes just chocka.