Green’s Dictionary of Slang

1. (UK Und.) a housebreaker [sneak v. (1)].

1628		R. Speed Counter-Rat E3: Constables [...] are Night-Sconces, out to seeke Night-Sneakers, Who late in Tauernes vp doe sit.
1708		J. Hall Memoirs (1714) 5: Sneakers, Such as Sneak into a House by Night or Day to Steal.
1877		S. James Vagabond Papers (3rd series) 136: At home, now, it’s the opposite—a good magsman wouldn’t sneak, or be seen with a sneaker.

2. a coward [they sneak about].

1805		‘Jeremy Swell, Gent.’ Tailors’ Revolt 12: And he’s a sneaker who don’t aid our cause!
1826		I. Pocock Woodstock I ii: I am glad he is no sneaker.
1838		D. Jerrold Men of Character I 252: A sneaker, take my word for’t.

3. (S.Afr.) a smuggler.

1963

L.F. Freed Crime in S. Afr. 107: A ‘sneaker’ is a smuggler.