Green’s Dictionary of Slang

1. the penis [the animal and the penis ‘burrow in’].

c.1850		Peeping Tom (London) 40 160/2: And the wanton little mousey / Goes — where I will not say. / The cat no more it fears, / For it burrows like a mole.
1890–1904		Farmer & Henley Sl. and Its Analogues.

2. (Aus.) in pl., moleskin trousers.

1901		Sydney Sportsman (Surry Hills, NSW) 23 Jan. 1/3: When a clean crimean shirt and white moles were full dress.
1904		Sun. Times (Perth) 26 June 4/8: They wunst they’d ’essian patches on / their moles an’ dungarees.

3. (US prison) someone who tunnels into a bank to rob it.

1931–4

D. Clemmer Prison Community (1940) 334/1: mole, n. A bank robber who tunnels under the vault.

4. (US prison) someone who escapes by digging their way out of prison.

1992

Bentley & Corbett Prison Sl. 108: Mole An inmate who escapes by digging his way out of the prison.