Gilbert Arenas' 43rd birthday was ass in the best way possible -- the ex-NBA star was surprised with his very own dancer right in the middle of his podcast ... with his fiancée present for the whole thing!!

The former Wizards great recorded an episode of his "Gil's Arena" show with Kenyon Martin, Brandon Jennings, Nick "Swaggy P" Young and Josiah Johnson on Monday ... and considering it was a celebration, the crew presented him with a cake -- and then more cake.

Arenas was caught off guard by the gesture, but understood the assignment when he was given a bag full of cash ... as he immediately started throwing bills at the special guest, who was rocking a rainbow-colored fishnet ensemble.

While the YouTube stream cut out for obvious reasons, the entire twerk sesh was caught on video by Arenas' fiancée, Melli Monaco, who was in full support of the gift.

"It ain't a real celebration if there is no dancer," Monaco said. "HBD!"

Arenas -- who proposed to Monaco last summer -- barely touched the dancer ... and made sure to keep things at a distance throughout the shakin'.

The guys explained they came up with the idea after Arenas revealed an old story from his playing days ... when he got around a team's curfew by bringing strippers to his hotel.

"Nothing like a fresh Azz shake at the work place 😂😂," Arenas said. "❤️Shake that azz for sum Cash 💰💯"

Happy birthday, Gil!!