Johnny Knoxville is a self-confessed jackass -- but says even he wouldn't knowingly bring a grenade into an airport.

In a statement on his website, Johnny says the wardrobe gal from a commercial shoot a few days back actually packed a "prop grenade" in his bag -- and he never knew it was there.

Law enforcement tells us Knoxville gave them the same story at LAX yesterday when he was stopped and cited for carrying a real grenade sans ammo.

Knoxville writes "Ah f**k was I bummed when I realized what had happened... They [the cops] were cool as they could be to me, considering the situation, and it is no one's fault but my own. I should have checked my goddamn luggage through and through."

Despite the hold up with security, he still made it to his flight on time and is now in Miami.