WTF! NAS OWES $6 MILLION IN BACK TAXES!

Nas is great at a lot of things ... rapping, feuding ... but paying taxes isn't one of them ... and now he's on the hook for another $339,000 he allegedly kept from Uncle Sam.

According to official documents obtained by TMZ, Nas never forked over $339,005.49 in federal income tax he owed for 2010 ... and now the U.S. government has obtained a lien on his property.

Being a tax scofflaw is nothing new for Nas ... according to public records, dude is also on the hook for more than $6 MILLION DOLLARS in back taxes from previous years.

That ni**a Nas is crazy! How the f**k you owe $6 million in taxes?