Green’s Dictionary of Slang

1. (orig. theatre, UK; Aus.) a word.

1960		J. Franklyn Dict. of Rhy. Sl.
1983		R. Aven-Bray Ridgey-Didge Oz Jack Lang 36: Mockingbird [...] word.

2. (Aus.) a piece of excrement [= turd n. (1)].

1983		R. Aven-Bray Ridgey-Didge Oz Jack Lang 36: Mockingbird Turd, word.
1990		Tupper & Wortley Aus. Prison Sl. Gloss. 🌐 Mocking bird. Faeces. Rhyming slang for turd.
1991		R.G. Barratt ‘I Saw That’ in What Do You Reckon (1997) [ebook] [H]e’d find a big, steaming mockingbird in amongst his schoolbooks.