Basically, "Yippee ki-yay" is an old, American cowboy expression, like: ”yippee”, or “yeehaw (heehaw)”, or “Whoopee”; expressions of extreme joy or excitement, commonly associated with cowboys. As an interjection, “yahoo”, plays a similar role.
PS.: Many people mention the expression “Yippee ki-yay” in the movie “Die Hard” as the origin of the work, but it is important to know that this word is much order. Like I said: from the “cowboys-days”.
Here is one example of use: I’ve been meaning to meet someone for, like, a month. Today, she finally was available. My reply? “Yippee ki-yay”, woman! Pick you up at eight.”
by Alexandre P Ramos - SP-BR November 4, 2015
This is the correct spelling, don't let anyone tell you otherwise. Commonly proceeded by the words mother and fucker and particularly used towards terrorists.
Watch Die Hard, where else you gonna find one?
by Josh Turnbull June 19, 2005
Catchphrase used by Bruce Willis In all of the Die Hard films.It only has context In the first movie but he says It In all of them.

The reason the phrase has context In the first one Is as follows:

Main terrorist Hans:Mr. Mystery Guest are you still there?

John Mcclane:Yeah I'm still here

Hans:Who are you exactly?Someone who watched too many action movies as a child and thinks he's John Wayne,Rambo,Lewis Stevens?

Mcclane:Actually I was kind of partial to Roy Rogers myself.Always liked those sequin shirts.

Hans:Do you really think you can stop us Mr.Cowboy?

Mcclane:Well,Yippie-Ki-Yay MotherFucker!
Do you really think you can stop us Mr.Cowboy?
Well,Yippee ki-yay, motherfucker!

by Metaknight42 July 9, 2007