Terrion Arnold's crummy January got a whole lot worse on Thursday ... when he took a scary tumble down a flight of stairs at his house.

Just days after the Detroit Lions cornerback lost a massive playoff game to the Commanders, he nearly hurt himself while heading down some steps ... and the fall was all captured on one of his home's video cameras.

Arnold actually posted the footage to his social media page ... and you can see, just as he gets to the top stair, he slips, and then slides the whole way down to the bottom.

Somehow, the 21-year-old -- a first-round pick in the 2024 NFL Draft -- avoided an injury catastrophe ... though check out the clip, he was clearly in disbelief over what had happened.

Arnold, though, was able to joke about the scene afterward -- captioning the vid "rough morning" with a bunch of crying laughing emojis.

It's good to see at least something bring a smile to Arnold's face these days ... as the Lions' Jan. 18 defeat to Washington is considered one of the worst in Detroit history.