Green’s Dictionary of Slang

lag n.2

[lag v.2 (1); note Aus. (Victoria) police jargon lag, ‘one who informs especially, though not exclusively, against his or her fellow-officers’ (Seal, The Lingo, 1999)]

1. [mid-18C–1900s] (UK Und.) a term of transportation or penal servitude.

2. [early 19C+] a convict who has been transported or sentenced to penal servitude.

3. [mid-19C] a convict who has finished his sentence or has been released on parole.

4. [mid-19C+] any convict; thus Lagland, the underworld.

5. [20C+] (prison) any length of sentence.

6. [1930s+] (Aus. prison) a three-month sentence [? link to tramp’s lagging under tramp n.].

In derivatives

lagdom (n.)

[late 19C-1900s] (Aus.) the pentitentiary world.

In compounds

lag fever (n.)

[early 19C] a spurious illness feigned in order to avoid transportation.

lag ship (n.)

[early 19C] a ship used for the transportation of convicts to Australia; a prison hulk.

In phrases