Weezy spilled the beans in a Bleacher Report interview with Taylor Rooks, who said she spoke with Durant before their sit-down ... and he let her in on their secret.

"He said he had a question -- 'Are you going to drop a snippet of you all song on the radio show?'" Rooks explained to Tunechi.

The "A Milli" rapper was caught off guard, as their duet was supposed to be "on the low" ... and Wayne hilariously called KD out for tellin' their business.

"I just want you to know you messed up," Wayne laughed as he sent a message to the Phoenix Suns superstar. "That song was being considered to be on 'Carter VI' and now since you're saying something, it's not goin' on 'Cater VI.'"

In fact, Wayne said it was initially supposed to go on the "Welcome 2 ColleGrove" album that just came out in November ... but he's saving their special song for something else.

"KD's a jewel," Wayne said about KD's rap skills. "He's a backpack rapper."

"I can imagine he probably could call a timeout in a game, and probably ask him to rap and he probably got a verse for you."

Of course, Durant isn't the only NBA player with bars ... Damian Lillard has been known to spit impressive lyrics on his songs, including his hit single dedicated to Kobe Bryant.

Even Shaquille O'Neal was a gifted MC ... and trendsetter Allen Iverson went by the rap name Jewels.

Durant has some experience in the rap game -- not only did he have the song "It Ain't Easy" with LeBron James, he was recently enlisted as A&R for Drake's "For All The Dogs" project.