50 Cent -- I Didn't Hijack That Song You Don't Own!

50 Cent doesn't own the 1970s song he recently sampled -- but in court docs obtained by TMZ, he claims the guy suing him for copyright infringement doesn't own it either!

As TMZ first reported, a man named Robert Poindexter sued 50 ... claiming the rapper hijacked a sample of his Persuaders song "Love Gonna Pack Up and Walk Out." Robert claims 50 used it on his track "Redrum," which he released for free online.

But in recently filed court docs, 50 says the suit is TOTAL B.S. and claims he never violated any copyright laws. In fact, he says Robert doesn't even own the rights to the allegedly-stolen song because he signed them over to Warner Music Group awhile ago ... therefore he can't sue.