Tom Brady Pissed Over NFL Future Question ... Drops F-Bombs

"If I knew what I was f***ing doing, I'd have already f***ing done it."

Tom Brady is fed up with everyone asking if he's going to retire ... 'cause the legendary quarterback just unloaded on his "Let's Go!" podcast co-host Jim Gray for trying to get an update on his decision.

The awkward moment went down as the two discussed the remaining teams in the NFL playoffs on Monday ... when Gray switched topics to Brady's future.

"Tom, you're leaving everybody guessing," Gray said. "You said you'll take your time. Do you have any type of a timetable as to what you might want to do regarding your football career?"

That's when Brady hurled his expletive-filled response ... clearly annoyed with having to…