‘Your father can barely talk!' Donald Trump Jr diagnoses Joe Biden with Alzheimer’s – and it blows up in his face
Donald Trump Jr. (Fox News)

President Donald Trump's eldest son tried to diagnose Joe Biden with Alzheimer's disease -- and social media users let him have it.

Donald Trump Jr. retweeted a video of Biden telling a man he was "full of sh*t" for claiming he wanted to end Second Amendment rights, and the president's son claimed that was evidence of cognitive impairment.

"From the Alzheimers association webpage," Trump Jr. tweeted. "'Aggressive or angry behaviors may occur in people with Alzheimer's or other dementias.'"

"A 50 year career politician doesn't snap back this way to a blue collar guy asking him a reasonable question given his parties stance on the issue," he added.

That launched a fusillade of fury and ridicule.