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1.
1) A guy who could use some more muscle.

2) A guy in the gym who is not very large and is called this by the juice users or gym rats.

3) A guy in the gym who walks around as if he were large - but isn't - and is called this by larger guys as a cut-down and derogation of his vanity.
1) "Mike is 6 foot 4 and 125 pounds. He needs to eat more; he looks like a friggin' bonerack."

2) Mick: "Look at that bonerack trying to press 180 pounds."
Jake: "Yea, he could use some help from our buddy Winni!"

3) Tom Cruise: "Move aside gents, I need to fix the machine. Me that is; I am the machine. Just measured the pipes yesterday; a solid 12 inches!"
Jake: - *purposely bumping into Tom and knocking him to the ground* - "Oh, sorry Mr. Bonerack Cruiser, I didn't see you there; you must have been standing sideways."
by psiscott2000 April 11, 2006
 
2.
An excessively (and obsessively) anorexic female.
That chick is such a bonerack that her pants are falling down.
by staccato brainstem February 12, 2005