Donald Trump -- Everyone Hates Kareem Abdul-Jabbar

Donald Trump just declared war on Kareem Abdul-Jabbar -- claiming he's a clueless bonehead ... and nobody likes him.

Seriously, this really happened.

Trump spelled out his issues with the NBA legend in a handwritten note -- after Kareem had penned an article for the Washington Post in which he called Trump a "bully" who's bad for America.

So, Trump being Trump -- responded in his own Trumpish way ... he wrote a note ON a copy of Kareem's article.

"Dear Kareem,

Now I know why the press always treated you so badly — they couldn’t stand you. The fact is that you don’t have a clue about life and what has to be done to make America great again!

Best wishes,

Donald Trump"

Kareem has already responded -- writing, "Trump’s response to my piece is the best, though inelegant, support for my claims.Here again, he attacks a journalist who disagrees with him, not by disputing the points made but by hurling schoolyard insults such as 'nobody likes you.'"