Green’s Dictionary of Slang

timer n.

[time n.]

(UK Und.) a criminal who has served time in jail; often in combs, e.g. first-timer under first adj., short-timer n. (1)

[UK]Leicester Chron. 14 June 12/1: The prisoner in the cell next to him was a bad character — a ‘third timer’.
[UK]S. Scott Human Side of Crook and Convict Life 19: There were three vanloads of us — young men and old men, novices and ‘timers’, all huddled in together.
[US]‘Boxcar Bertha’ Sister of the Road (1975) 145: I just got to beat this rap [...] I’m a four-timer, and if I’m convicted I’ll be sent to the penitentiary for life.
[US]Monteleone Criminal Sl. (rev. edn).
[SA]A. Dangor Z Town Trilogy 150: Gwan, you snotkop, don’t lol with timers .