Green’s Dictionary of Slang

lagger n.2

[lag v.2 ]

1. a convict.

[UK]Sporting Mag. III. 230/2: The laggers had an interest as to the result .
[UK]Egan Anecdotes of the Turf, the Chase etc. 288: I beg pardon, Bob and brother laggers.
[UK]W.T. Moncrieff Heart of London II i: craig: Lagger Jack. jack: Here.
[Aus][A. Harris] (con. 1820s) Settlers & Convicts 13: The blue jacket and trousers of the English lagger.

2. (Aus. 20C+) a police or prison informer.

[UK]Sl. Dict. 212: Lagger [...] one who gives evidence; an informer.
J. McNeill Chocolate Frog (1973) 21: tosser: [T]had weak bludger Brown [...] He used ter be a lagger at Gosford.
[Aus]B. Ellem Doing Time 130: Rapes do go on but the guys are too frightened to go and report it because [...] [i]f they do they will be tagged a lagger and someone will get them later on.
[Aus]Tupper & Wortley Aus. Prison Sl. Gloss. 🌐 Lag. 1. To inform. [...] a lagger is an informer.

3. (Aus.) a policeman.

[Aus]W.A. Sun. Times (Perth) 1 Dec. 1/1: A Fremantle p.c. constable has appointed himself adviser-in-chief to females in affluence [...] The lanky lagger advised a departing demoiselle to abide with him and miss the boat.

4. an ex-convict.

[US]Howsley Argot: Dict. of Und. Sl.
[US]Monteleone Criminal Sl. (rev. edn).

5. (Aus. prison) a recidivist.

[Aus]B. Ellem Doing Time 192: lagger: an old prisoner who has done several sentences or laggings; he is usually referred to as an ‘old lagger’.

In compounds

baby lagger (n.)

(N.Z. prison) an inmate serving a short prison sentence (usually their first) .

[NZ]D. Looser Boobslang [U. Canterbury D.Phil. thesis] 103/1: baby lagger n. n inmate serving a short prison sentence (usually his first sentence).
first lagger (n.)

(N.Z. prison) a gullible, exploitable inmate .

[NZ]D. Looser Boobslang [U. Canterbury D.Phil. thesis] 103/1: first lagger n. 2 an inmate who is gullible, an easy mark, easily conned.
jet lagger (n.)

(N.Z. prison) one who serving a very short sentence.

[NZ]D. Looser Boobslang [U. Canterbury D.Phil. thesis] 103/1: jet lagger n. an inmate serving a very brief sentence.
little lagger (n.)

(N.Z. prison) one who serving a very short sentence.

[NZ]D. Looser Boobslang [U. Canterbury D.Phil. thesis] 103/1: little lagger n. an inmate serving a short sentence.
long lagger (n.)

(N.Z. prison) one who is serving a long sentence, usu. 7 years or more; one who has completed the majority of that sentence.

[NZ]G. Newbold Big Huey 107: I just slipped [...] into the main clique in the prison, which was dominated by the long-laggers.
[NZ]D. Looser Boobslang [U. Canterbury D.Phil. thesis] 103/1: long lagger n. an inmate serving a long sentence (usually over seven years) esp. an inmate who has served a considerable amount of that sentence.
short lagger (n.)

(N.Z. prison) one who is serving a short sentence.

[NZ]D. Looser Boobslang [U. Canterbury D.Phil. thesis] 103/2: short lagger n. an inmate serving a short sentence.
violent lagger (n.)

(N.Z. prison) one who has committed some form of violent crime.

[NZ]D. Looser Boobslang [U. Canterbury D.Phil. thesis] 103/2: violent lagger n. an inmate sentenced for an offence involving violence, e.g. armed robbery, aggravated robbery, assault, or murder.