The act of waking someone up by sticking one's finger in one's rectum, and then shoving said finger into the nostril of the passed out/sleeping individual.
After Tom passed out at the party, John decided to wake him up with a rusty knuckle.
Similar in VERY FEW regards to "Bloody Knuckles" that we played as kids....
"Rusty Knuckles" is simply (depending on your particular 'acceptance' of said limb) the act of anal fisting.
does this really need an example?
I guess....see goatse.cx for historic reference?
"Oh man, Justin and Andrew were having playtime.....Justin was stretching for his receiving of Andrews clenched fist. I bet he has Rusty Knuckles now!"