Lil Wayne -- To Be Blunt ... What's He Smokin??

Now we're not sayin' Lil Wayne got stoned at his 30th bday party in Hollywood on Thursday ... but the guy was definitely smokin' something that bares a striking resemblance to a fatty blunt ... and TMZ has the footage.

Now, if ... and ONLY IF ... Wayne WAS puffin' on weed, it could spell BIG TROUBLE for the rapper ... because he's still on probation from a 2008 drug and weapons case ... and he's been ordered to keep his nose clean.

Of course, since Weezy is only on informal probation and probably won't be tested for drugs anytime soon, there's really no way anyone can know for sure what Weezy was smokin' that night ... except Weezy.

In other words -- no worries.