Green’s Dictionary of Slang

1. a ne’er do well.

1771

Smollett Humphrey Clinker (1925) I 216: He is also blest with an only son, about twenty-two, just returned from Italy, a complete fiddler, and dilettante.

2. a cheat, a swindler.

[1659		Greene & Lodge Lady Alimony V iii: You shall play no more the sharking foist with me, you fumbling Fidler you].
1857		‘Ducange Anglicus’ Vulgar Tongue.
[1859		Matsell Vocabulum 126: fiddler. A pugilist that depends more upon his activity than upon his bottom].
1860		Hotten Dict. of Modern Sl. etc. (2nd edn).
1873		Sl. Dict.
1895		Crowe Aus. Sl. Dict. 28: Fiddler, [...] a cheat or sharper.
1946		S. Jackson Indiscreet Guide to Soho 113: A man may come in with a violin case but he is often a ‘fiddler’ in an unmusical way.
1963		N. Dunn Up the Junction 120: Me mates are all fiddlers.
1979		J. Morrow Confessions of Proinsias O’Toole 138: It would be safe with a time-served fiddler like Punchy.

3. (UK und.) a thief.

1935

Thieves Slang ms list from District Police Training Centre, Ryton-on-Dunsmore, Warwicks 4: Fiddlers: Thieves.

4. see kiddy-fiddler under kiddy n.