The well-established, but long forgotten, rule of law that you will honour your obligations and treat your counterparty to a deal with the level of deference and respect due to them as though they had a compromising photograph of you with a donkey.
Tim: Hey Vinnie, how'd you go getting out of that crap million dollar deal you'd struck with KK?
Vinnie: No good mate, I forgot about the Donkey Principle and I had to suck it up and pay him his loot.
When a male (ideally) is "rooting" his beloved in the anal cavity, strikes her on the back of the cranium, in order to induce clenching of the chocolate starfish... He then completes the manouvere by wiping the anal residue on her upper lip... He then tells his mates and earns brownie points....
A sexual act wherein the adult male places luggage on the adult female's back before mounting her doggy style.
Ryan: "Hey Rico, have you seen my Sampsonite suitcase?"
Rico: "Oh yeah, I put it on my wife last night."
Ryan: "Nice! Donkey Style!"
Rico: "Yeah, Zazzy let's me do whatever I want on Friday nights."
Ryan: "TGIF, my man!"
Rico: "Desaint!" (See urban dictionary definition for "desaint")