"Blame it on the a-a-a-a-a-alcohol" -- Jamie Foxx, T-Pain, and now, Paul Pierce ... 'cause the Celtics legend is opening up on his infamous stripper video that led to his ESPN firing -- saying he was boozin' hard the night he accidentally streamed his scandalous party on Instagram.
Pierce gave Lance Armstrong the lowdown on the whole controversy in the latest episode of the hit Fox celebrity competition show, "Stars on Mars" ... and he downplayed the whole incident -- even though he lost his TV gig over it.