1:30 PM PST -- Sources close to both T.I. and Drake say the culprit was a heavily intoxicated friend of T.I.'s who peed himself. The pee unintentionally got on Drake's shoe and that's when he got pissed.
The lyric in question came at the very end of "Wanna Know," where Meek said, "You let Tip homie piss on you in a movie theater n****, we ain’t forget."
Our sources say, it all went down in 2010, during a private screening of "Takers" on the Sony lot ... a movie in which Matt Dillon, Paul Walker, Chris Brown and T.I. all starred in.
Drake attended the screening and, 30 minutes in, jumped up screaming and ran out of the theater.
We're told a dispute erupted over who was sitting where ... apparently Drake and one of T.I.'s childhood friends got into it. The friend got pissed, and pissed on Drake.
We're told Drake ran out of the theater, screaming "motherf*****."
And sources told us the obvious ... intoxicants were involved.