Shaquille O'Neal Admits No. 2 Emergency Sparked Abrupt 'Inside The NBA' Exit

Shaquille O'Neal is clearing the air on his infamous bathroom emergency on "Inside the NBA" last month ... admitting once and for all he had to take a dookie.

Yes, someone is getting paid to write this.

The NBA legend let it all out during his appearance on "The Tonight Show" with Jimmy Fallon on Thursday -- after initially insisting his rush from the TNT desk was to drain the snake, he revealed it was a dire situation involving clenched butt cheeks.

53-year-old Shaq started off his explanation by saying he drank a lot of water that day ... and even though he displayed a "No. 2 run," it was not how it seemed.

But the four-time champ couldn't keep up with the lie ... and told Fallon he really…