No definite origin is known but the idea is given in the episode "My Comedy Show" where J.D. says that on the anniversary of him and Turk meeting David Caruso on a ferris wheel, Turk would lift J.D. in the air and he would straddle him and yell "Eagle!". He also states that it was the fifth anniversary. This cannot be possible since the first eagle takes place in "My Hero", seven years prior.