Green’s Dictionary of Slang

an ‘inevitable’ nickname for someone surnamed ‘Smith’.

1959		W. Hall Long and the Short and the Tall Act I: smith: When you start giving us the heels together I’ll be commanding the Camel Corps. bamforth: Get your bucket and spade, Smudger, and I’ll lay it on.
1981		P. Wright Cockney Dialect and Sl. 92: Thus Toffee Taylor (note too the alliteration, as so often like Smudger Smif.