A supposed "high" attained by release of endorphins during and after high amounts of aerobic activity. Possibly developed in the 1960's as a way to get the drug addicts of the class to exercise. Current research indicates no high is ever achieved via this method, and so can efficiently labeled "Bullshit".
Fred: Hey man! Lets go for a run and get a runner's high!
Me: You're an idiot. Go take some LSD.
Fred(tripping): OH MY GOD!!!! THE COLORS ARE TALKING TO ME!!!!