Green’s Dictionary of Slang

-rat sfx

(US) constructed with a noun to indicate someone’s habit or particular preference e.g. arcade rat, gym rat, mall rat n.

[Aus]E. Dyson ‘Susie Gannon’s Young Man’ in Benno and Some of the Push 113: ‘Garn!’ he cried, ‘[...] le’ the poor woman be, carn’t yeh? ’Ave a bit iv manners if y’ are fact’ry rats’.
[UK]I. & P. Opie Lore and Lang. of Schoolchildren (1977) 211: Grease rat, shan’t play with us.
[US]Current Sl. III:1 7: Grill rat, n. A student who spends most of his time in the student union grill.
[Aus] in Tracks (Aus.) Mar. 5: I’m a surf rat [...] cause that’s what I live for.
[US]Wall Street Journal R-7/1: With time to kill and money to spend, teen-age ‘mall-rats’ can’t stay away.
[US]B. Hamper Rivethead (1992) 4: His daddy was a shoprat. His daddy’s daddy was a shoprat. Perhaps his daddy’s daddy’s daddy would have been a shoprat if only Hank Ford would have dreamed this shit up a little sooner.
[US]T. Jones Pugilist at Rest 70: After my day of isolation, a brig rat, a white marine named Rouse, came up to me.
[US]Eble Campus Sl. Apr. 7: -rat – a frequenter of. Club-rat [...] frat-rat and gym-rat.
[UK]Guardian G2 16 July 12: He was elevated to the status of love-rat-in-chief.
[UK]Guardian Weekend 4 Sept. 17: You take a serious gym rat, a man who lives in a gym.
[UK]Guardian Rev. 11 Feb. 6: Rookie wave-rider Keanu Reeves breaches a cardinal tenet of surf-rat etiquette.