Green’s Dictionary of Slang

1. a diligent student.

1901

Monroe & Northup ‘College Words and Phrases’ in DN II:iii 139: digger, n. A grind.

2. (US Und.) a pickpocket.

1931		G. Milburn ‘Convicts’ Jargon‘ in AS VI:6 438: digger, n. The modern term for pick-pocket.
1949		Monteleone Criminal Sl. (rev. edn).
1950		Goldin et al. DAUL 58/1: Digger. A crude pickpocket; a dig.
1960, 1975		Wentworth & Flexner DAS.
1996		Eble Sl. and Sociability 51: Baby raper and chicken hawk mean ‘child molester’; crib burglar, ‘someone convicted of breaking and entering a domicile’; and digger, ‘pick-pocket’.