Word to the wise ... if you see Jessie "Mr. PEC-Tacular" Godderz on the streets, DON'T challenge him to a thumb wrestling match, or your butthole might just pay the price.

Our TMZ camera guy learned that the hard way in Manhattan, when he faced off against the jacked "Big Brother" legend in a battle of opposables ... and LOST.

What was on the line? Loser gets a wedgie.

Well, our guy lost ... and the punishment was so bad, Jessie literally ripped the photog's undies as he pulled 'em over his head.

Ice up, son.