Matt Barnes thinks it's FUNNY AS HELL that Paul Pierce used a wheelchair to take a dump in that infamous 2008 NBA Finals game ... telling TMZ Sports, "That's funny as a mother f*cker."
If ya missed it ... Pierce came clean on ESPN's Finals pregame show Wednesday night about why he needed a wheelchair to get off the court in that Celtics-Lakers series, saying, "I just had to go to the bathroom."
When we got Barnes out in Hollywood just hours later ... he couldn't stop laughing at Pierce's revelation.
"I never knew why he went out like he got shot and he came back like Superman, I was just like damn," Barnes says ... "But, he sh*tted. That's crazy. It takes a hell of a man to admit that sh*t, though, man."
"That's funny as a mother f*cker."
As for how Barnes would have handled the crappy situation? He tells us he definitely wouldn't have used a wheelchair to get to the toilet ...
"I would have slid on my ass all the way back to the locker room. Hopefully, I didn't make a streak on the court."