Brendan Schaub is putting his dong on the line for Mayweather vs McGregor﻿ ... promising he'll DM the world his finest wiener pic if Conor doesn't go all 12 rounds with Floyd.

That's right -- the UFC fighter-turned-MMA analyst is so sick of fans countin' out the Irishman ... he's bettin' his manhood on McGregor's chin.

"I promise you Conor will not get knocked out," Schaub told Rob King on "The Kicker" ... "If anyone gets finished that night. It's Floyd."

King asked Brendan if he was willing to put a wager on it -- to which Schaub replied, "I will send you guys a d*ck pic if it doesn't go all 12 rounds."