Green’s Dictionary of Slang

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

1911		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’.
1959		I. & P. Opie Lore and Lang. of Schoolchildren (1977) 211: Grease rat, shan’t play with us.
1968		Current Sl. III:1 7: Grill rat, n. A student who spends most of his time in the student union grill.
1985		in Tracks (Aus.) Mar. 5: I’m a surf rat [...] cause that’s what I live for.
1988		Wall Street Journal R-7/1: With time to kill and money to spend, teen-age ‘mall-rats’ can’t stay away.
1991		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.
1993		T. Jones Pugilist at Rest 70: After my day of isolation, a brig rat, a white marine named Rouse, came up to me.
1997		Eble Campus Sl. Apr. 7: -rat – a frequenter of. Club-rat [...] frat-rat and gym-rat.
1999		Guardian G2 16 July 12: He was elevated to the status of love-rat-in-chief.
1999		Guardian Weekend 4 Sept. 17: You take a serious gym rat, a man who lives in a gym.
2000		Guardian Rev. 11 Feb. 6: Rookie wave-rider Keanu Reeves breaches a cardinal tenet of surf-rat etiquette.
2024		C. Stella Joey Piss Pot 37: Rapino was a dedicated gym rat who used almost all of the equipment [...] 3 to 4 times a week.