David Spade -- Ronda Rousey In The WWE ... Is a TERRIBLE Idea!

David Spade thinks Ronda Rousey should say buh-bye to all WWE action for the sake of her career -- but he's got a theory about why she keeps pushing to be a pro wrestler.

Spade -- who's a big fan of the UFC and Ronda -- was out in L.A. when he told us what he thought about Ronda saying she's down to be the next big thing in turnbuckle tussling.

FYI -- the WWE has a long history of having MASSIVE stars from the combat fighting world throw down with them, including Floyd Mayweather Jr., Mike Tyson and Muhammad Ali himself.

David says Ronda would be taking a "backwards" move if she followed in those footsteps. As for why she wants to do it -- he thinks it has a lot to do with how much ass she's kicking in the Octagon.