Lil Wayne's Back Taxes -- The Only Lien I Care About is in a Double Cup

Lil Wayne has not only stomped on the American flag, he's now said F you to Uncle Sam, because we've learned he hasn't paid his taxes -- TO THE TUNE OF MORE THAN 12 MILLION BUCKS!

According to a tax lien -- obtained by TMZ -- Weezy failed to pay taxes for 2011 and 2012. For 2011 he owes a staggering $5,843,952. And for the following year, even worse -- $6,311,132.

Wayne has had his financial issues. He owed $7 mil for back taxes in 2008 - 9, but he cleared the debt in 2012.

He's also had legal issues. In 2012 ... Wayne famously lost a case against Quincy Jones' son after thumbing his nose at the court. A jury socked Wayne for $2.2 mil.

Wayne is reportedly worth $135 MILLION.