Wee Man Disses Justin Bieber ... You're No Skateboarder!

Get ready for some epic tiny man beef ... 'cause world famous skater Wee Man is taking SHOTS at Justin Bieber's skateboarding skills.

Wee Man is a skate aficionado who's been rollin' with the "Jackass" guys for years ... so, when we saw him cruisin' through LAX with his board, we had to ask what he thought of Bieber's skills.

"He's like a fruitcake singer ... how can he be a skater??"

FYI, "fruitcake" has different meanings in different circles -- unclear how Wee Man meant it.

Still, might not be too late to say sorry ...