dialogue - What does "yippie ki yay" mean?

An article on The Week.com broke down each part of the phrase:

...But where does the yippee-ki-yay part come from?...Let's break it down.

The yip part of yippee is old. It originated in the 15th century and
meant "to cheep, as a young bird," according to the Oxford English
Dictionary (OED). The more well-known meaning, to emit a high-pitched
bark, came about around 1907, as per the OED, and gained the
figurative meaning "to shout; to complain."

Yip is imitative in origin but probably also influenced by the 16th
century yelp, which has an even older meaning of "boasting,
vainglorious speaking." Yawp is even older, coming about in the 14th
century, but now is primarily associated with Walt Whitman's late 19th
century "barbaric yawp."

The yips are "nervousness or tension that causes an athlete to fail to
perform effectively, especially in missing short putts in golf." As we
mentioned in a Word Soup column back in November, some sources,
including the OED, cite the first known use of the yips as 1962.
However, we found a citation from 1941: "The match consumed three
hours and thirty minutes, most of it because Cobb, the tingling-nerved
old baseball Tiger, got the 'yips' on many greens and would step back
and line up his putts several times per putt."

Yippee came about after yip. The earliest record of this exclamation
of delight is from 1920 in Sinclair Lewis's novel, Main Street: "She
galloped down a block and as she jumped from a curb across a welter of
slush, she gave a student 'Yippee!'" Yippee beans, by the way, are
amphetamines.

Yippie with an -ie refers to "a member of a group of politically
radical hippies, active especially during the late 1960s." The word,
which originated in 1968, stands for Youth International Party and was
modeled after hippie.

Now how about the whole phrase, yippee-ki-yay? It seems to be a play
on "yippie yi yo kayah," a refrain from a 1930s Bing Crosby song, "I'm
An Old Cowhand."

Do cowboys really say this? We're guessing probably not, unless of
course they're single-handedly (and shoelessly) defeating a gang of
bank robbers on Christmas Eve.

So that's some explanation as to its origin. It's worth pointing out though that Bruce Willis had this to say (in an interview with Ryan Seacrest) about the phrase:

On The Famous “Die Hard” Line Yippee-Ki-Yay…

“It was a throwaway. I
was just trying to crack up the crew and I never thought it was going
to be allowed to stay in the film.”