Tom Brady Epically Bungles 4th Down On Game-Winning Drive, 'What Down?!'

Did Tom Brady forget what down it was in a crucial moment during the Bucs game Thursday night???

It certainly seems so ... 'cause cameras caught the QB asking refs what down it was after he had just thrown a game-losing incompletion on 4th down!!

Here's the deal ... after the Chicago Bears kicked a field goal to go up 20-19 on TB12's Bucs late in the 4th quarter -- Brady took the ball with less than 90 seconds left in the game.

Tom looked poised to lead Tampa Bay to its 4th win this season and was driving ... when, suddenly, his offense started to sputter.

On 1st down, Tom missed Mike Evans. On 2nd down, #12 hit his running back for a short gain. On 3rd down, Brady's pass to Rob Gronkowski was broken up. And, on 4th, the QB threw another incompletion.

But, as his whole team was walking off the field -- Brady looked dumbfounded, held up 4 fingers and appeared to ask the ref, "What down?!"

The debacle cost the Buccaneers the game ... 'cause the Bears took a couple knees and ended the contest.

Afterward, Brady was asked about the down-counting disaster -- and he claimed he actually knew it was 4th.

"I knew we had to gain a chunk," Brady said, "so I should have been thinking more 1st down instead of chunk in that situation."

Added his head coach, Bruce Arians, "Yeah, he knew."

Sure, guys. Suuuuuuuuuuuuuure.