old lag
noun
1
a regular prisoner or one who has become institutionalised, a recidivist. From LAG (a prisoner) UK, 1950.
2
a prisoner who has been in jail for a long time AUSTRALIA, 1950.
3
a former prisoner AUSTRALIA, 1812.
4
a person who has been contracted to a single employer for a very long time, especially of the armed services. Humorous use of the sense as 'a convict who has been imprisoned for many years' UK, 2001