Yeah, that happened Thursday night during 45's rally in New Hampshire ... where there were a number of protesters tossed from the event. Trump decided to take a cheap shot at the size of one of them, riffing from the podium ... "That guy's got a serious weight problem. Go home, start exercising!"

Of course, the crowd roared with approval at the massively ironic jab from the Prez -- but the joke was ultimately on our fast food-loving Commander-in-Chief. Turns out the guy he was ripping was a Trump supporter!

His name's Frank Dawson and, according to Fox News, he's a Navy vet who's also been in law enforcement for 31 years.

Frank didn't seem the slightest bit offended, and says he was actually ripping up signs held by protesters when Trump mistook him for one of them.

For what it's worth, Frank seems to be in wayyy better shape than Trump -- but, clearly, logic or rationale played ZERO part in the President's remarks.