Green’s Dictionary of Slang

1. (US campus) a diligent or over-dedicated student, one who studies hard [dig v.¹ (1), i.e. they dig for knowledge. The term also flourished briefly in UK schools in the late 19C].

1830		[Harvard] Collegian 231: I could see [...] the many honest digs who had in this room consumed the midnight oil.
1845		J. Pycroft Collegian’s Guide 231: I could see, in the long vista of the past, the many honest digs who had in this room consumed the midnight oil.
1856		B.H. Hall College Words (rev. edn) 159: dig. A diligent student; one who learns his lessons by hard and long-continued exertion.
1863		Yale Literary Mag. xxviii 199: A ‘dig’ may be at times a genius, but a genius can never be a ‘dig.’.
1871		L.H. Bagg Four Years at Yale 44: Dig, a close, mechanical student.
1895		J.S. Wood Yale Yarns Preface: If you describe the life of the ‘rowdy element,’ you may offend the ‘digs’.
1902		Ade Forty Modern Fables 254: He cooped himself up in his Room and became a Dig and soon enough was greatly despised as a Pet of the Professors.
1915		M.G. Hayden ‘Terms Of Disparagement’ in DN IV:iii 199: dig, an over-diligent student.

2. (US tramp) a hiding place for stolen goods [? one digs a hole for the cache].

1927		‘Jargon of the Und.’ in DN V 444: Dig, A dump where stolen goods are hidden.
1931		Irwin Amer. Tramp and Und. Sl. 61: Dig. – A hiding place for stolen goods, probably from the fact that the old yeggs not infrequently laid their spoils away under the ground.

3. (US Und.) a pickpocket.

1950

Goldin et al. DAUL 58/1: Dig, n. An unskilled pickpocket.

4. (drugs) an injection of a narcotic [the needle ‘digs into’ the vein].

1993

I. Welsh Trainspotting 16: One wee dig tae unravel those twisted limbs and send us oaf tae sleep.