President Biden Told 'Let's Go Brandon' During Christmas Eve NORAD Call

President Biden heard what his haters have been saying in code for months now -- a "f*** you" jab that one guy had the gall to say right to POTUS' face, on Christmas Eve no less.

JB and the First Lady were making calls to military families and their kids Friday from all over the country ... talking Christmas, presents, wish lists and of course, Santa Claus. In fact, 46 and the missus were using the trusty NORAD Santa tracker to help in their discussions.

During one of the calls ... a father hopped on, and wished the Bidens a Merry Christmas. All seemed fine until his last words before hanging up ... as he clearly said, "Let's go Brandon."

Joe and Dr. Biden didn't seem to understand what it really meant, especially Joe -- who actually repeated it word for word himself and added, "I agree." Both even chuckled upon hearing it, which doesn't make sense on its face ... y'know, since it's super derogatory.

You probably know by now ... the phrase "Let's go Brandon" has become a joke in conservative circles. It actually means "F*** Joe Biden" ... and got flipped into 'LGB' because of a reporter's attempt to cover up what a NASCAR crowd was chanting during a live shot.

Even though the guy who unintentionally spurred the meme, Brandon Brown, has said he's not a fan of it at all ... that hasn't stopped lots of people from adopting it and using it in several different settings ... like on the intercom of an airplane, for example.

Some have called this a slur, and highly disrespectful -- but in light of Donald Trump receiving similar treatment from his own opposition throughout his presidency, others say it's fair game.