Green’s Dictionary of Slang

slug and snail n.

a (finger)nail.

1886		Sporting Times 20 Mar. 1/2: The graft she put in with her ten slugs and snails / Made the gore fly all over the place.
1992		R. Puxley Cockney Rabbit.
2002		B. Kirkpatrick Wicked Cockney Rhy. Sl.