Green’s Dictionary of Slang

Covent Garden n.

a farthing.

1859, 1860		Hotten Dict. of Modern Sl. etc.
1873		Sl. Dict.
1881		Morn. Bulletin (Rockhampton, Qld) 18 July 2/6: For a farthing there are a ‘Covent Garden’ and a ‘fadge.’ A ‘fiddler’ is used for a farthing as well as a sixpence, and the same explanation does for both. ‘Gig’ is a corruption of the word ‘Grig,’ and ‘Grig’ itself is a slang word for a merry little fellow.
1937		Nott. Eve. Post 30 Apr. 6/3: Now we come to the humble farthing [...] it was soon turned into rhyming slang into ‘Covent Garden’. Other names are ‘quartareen’ [...] ‘fadge,’ ‘grig,’ and ‘fiddler’.