An extremely fit individual who is always at the gym working out. They combine their high load basic exercises with new, complex shit that nobody has ever seen before.

Most often times gym ninjas are guys. However, there have been rare sightings of hot and fit women 'gym ninjettes.'
Everytime I go to the gym, I see the same gym ninja doing that weird cardio ab workout on the half blue ball.
by JimminyJillikers December 3, 2008
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