Used as a insult or a friendly come back.
It's technically impossible to suck a monkey's asshole, so don't attempt it. The phrase "suck monkeyhole" is said to someone that is so full of shit, that they should go suck the shit out of a monkey's asshole to keep a steady supply.
*Literal translation for "suck monkeyhole": "You are such an asshole, whether it's intentional or not. For your own good, you should take a straw, shove it up a monkey's asshole, and suck real hard. Make sure you get as much shit as possible, because you're spitting so much out now that you may be running low on your shit-supply. I'm only telling you this because I love you."