Young Thug's fans need to hit the gym -- 'cause they failed miserably when the rapper tried to crowd surf ... but ended up falling upside down in the chaos.
Thug and 21 Savage had the crowd at Terminal 5 jumping Sunday night, but things took a turn when Thug hurled himself into the sea of fans ... no doubt, expecting them to have his back.
Here's the thing ... Thug can't be more than a buck 60, dripping wet, but you gotta see how the crowd struggled, and eventually just dropped him on his head. It took several minutes, but security eventually dove in too, and rescued him.
This was the last show of the tour, and Thug went out with a bang. On his head.