Green’s Dictionary of Slang

1. (US, also gym bunny) a sports enthusiast; usu. one who frequents gyms and training grounds; often used of young gay men obsessed with body building.

1978		Wash. Post 30 Apr. D1: He plays volleyball, softball, ‘gym-rat basketball,’ and runs [HDAS].
1995		G. Pelecanos Down by the River 81: ‘You look at ‘em, weight lifters, gym rats, with the sideburns and the pompadours, they all look like young Elvises’.
2005		E. White My Lives 223: The gym bunnies living around us in Chelsea weren’t aware of my existence. [Ibid.] 229: His buttocks weren’t tight and tiny and rock-hard like the usual gym rat’s.
2012		(con. 1980s) I. Welsh Skagboys 63: A heavy-muscled gym rat, he rarely drank.
2012		M. McBride Frank Sinatra in a Blender [ebook] Then that guy’d take the ’roids to a gym, distribute to all the gym rats.
2017		V. McDermid Insidious Intent (2018) 224: His weren’t just gym-bunny muscles.

2. (US) in fig. use, anyone fanatically pursuing a course of activity, a career etc.

1993

Lerner et al. Dict. of Today’s Words 85: Gym rat – A person whose intense efforts and long hours demonstrates an intense determination to win at all costs. A Time magazine article on Jay Leno described him as ‘the gym rat of comedians’.