A “gym bunny” is someone who spends a lot of time at the gym. A “gym bunny” differs from a “gym rat” in that the gym rat is a dedicated individual to the sport of bodybuilding, but a gym bunny is more of a sexual show-off. “Gym bunny” has been cited in print since at least 1993 to describe the muscle-building efforts of homosexual men, but since the 2000s, “gym bunny” has also applied to heterosexual men and to women.
 
An attractive woman at the gym is often called a “gym bunny” (gym + Playboy bunny).
 
 
Wiktionary: gym bunny
Noun
gym bunny (plural gym bunnies)
1. (idiomatic, colloquial, bodybuilding) A person who spends a large amount of time working out at a gym and who may be obsessed with improving his or her physique. Often said of a gay man, but also said of women and heterosexual men.
 
The Free Dictionary
gym bunny
n
(Individual Sports & Recreations / Gymnastics) Informal a person who spends a lot of time exercising at a gymnasium
   
Urban Dictionary
gym bunny
A gym bunny is someone who spends large amounts of time working out for the sole purpose of ‘looking ripped’. Can be male or female, always extremely vain and superficial. Prevalent in California, which should come as no surprise since this state is a magnet for all the nation’s nutjobs and screwups.
 
The difference between a gym bunny and someone who just works out a lot is the latter exercises to be able to do physical stuff like move heavy objects on a regular basis, whereas the gym bunny does it exclusively for visual reasons and probably couldn’t do anything with those perfectly toned muscles if they had to.
(...)
by C++ Jun 3, 2006
 
Google Books
Routledge Dictionary of Modern American Slang
By Tom Dalzell
New York, NY: Routledge
2008
Pg. 473:
gym bunny noun
a male homosexual devoted to physical conditioning, US 1993
 
Google Books
Under My Skin
By Mary Di Michele
Kingston, Ont.: Quarry Press
1994
Pg. 194:
“Don’t you know it’s not fashionable to dress like a gym bunny anymore?”
“Is that right? What? Did you read that in Vogue?”
 
Village Voice (New York, NY)
Babes In Boyland
Pumping it up in the Pines
By Guy Trebay
Tuesday, Aug 17 1999
It started with the clone. Crew cut, plaid flannel, faded button-fly Levi’s with a sandpapered crotch. And let’s not forget the unfortunate mandatory upper-lip hair. The clone begat the polo queen (button-down with sport shirt underneath, collar turned up), who in turn begat the act up punk boy (button-down with frayed collar, ratty corduroy jeans), who begat the prototypical gym bunny, early versions of which would by now resemble the proverbial 98-pound weakling.
   
ABC (film reviews)
The Scorpion King
director: Chuck Russell
country: USA/WWF
release: 18/4/02
official website
rated: 3/5
review date: 17/04/2002
cast: Dwayne “The Rock Johnson
(...)
A classic underdog figure, Mathayus garners the support of rogue tribes of B.C. warriors after they discover he’s really a sweetheart under all that gym-bunny muscle.
 
Google Books
Re-Energize Your Relationship:
Reignite the Spark And Keep It Burning
By Sabina Dosani and Peter Cross
New York, NY: Perigee Books (Penguin Books)
2005
Pg. ?:
Regular strength training gives you a sexier body and raises testosterone. And you know what that means. We realize not everyone’s a gym bunny.
   
Google Books
Gay-2-Zee:
A Dictionary of Sex, Subtext, and the Sublime
By Donald F. Reuter
New York, NY: St. Martin’s Press
2006
Pg. 99:
gym bunny • One who is there constantly, usually attractive and groomed, very social, bounces between old and new friends, and equipment. (“Sister of a “disco bunny.”) (AKA Muscle Mary)
   
The Daily Progress (Charlottesville, VA)
Being a gym rat not the same as a gym bunny
By: Jenny Gardiner
Published: December 05, 2010
I’ve become a gym rat, which is a good thing.
 
I say gym rat, not bunny, because there is a serious distinction between the two — and, sadly, I’ve aged out of the gym bunny phase.
 
Not that I ever was one, mind you. Gym bunnies are those gorgeous, svelte young women who turn heads at the gym even when they haven’t showered and are drenched in sweat.
 
Davey Wavey Fitness
Gym Slang & Terms Every Exerciser Should Know!
June 14, 2011 By Davey Wavey
(...)
Zeb says:
June 14, 2011 at 6:58 pm
You didn’t define Gym Bunny.
What do we call the jerks who just take up space in the gym and don’t actually do anything? They can usually be found leaning against the piece of equipment that you need.
(...)
Rene says:
June 15, 2011 at 7:00 pm
gym bunny….. you see them every single day including almost all holidays
 
Daily Mail (UK)
Gym bunny Mila Kunis hops to yet another exercise session as she prepares for new movie role
By IONA KIRBY
PUBLISHED: 13:45 EST, 22 August 2012 | UPDATED: 01:47 EST, 23 August 2012
She famously shed 20 pounds to play a ballerina in Black Swan and Mila Kunis is once again putting her all into a film role.
 
The actress has been working out on a near daily basis to get her body in great shape for Jupiter Ascending.