Green’s Dictionary of Slang

lag v.2

also leg
[16C SE lag, to carry off, to steal]

1. [mid-18C–19C] to sentence to transportation for more than seven years.

2. [early 19C+] (also lag off) to arrest, to apprehend.

3. [mid-19C] (UK Und.) to cause trouble for; to lead to an arrest.

4. [mid-19C+] to imprison.

5. to cause someone to be arrested / imprisoned.

6. [late 19C+] (Aus.) to inform on.

7. [1910s] (Aus. / US Und.) to imprison on trumped up charges and faked evidence.