His first thought was giddyup, and in fact that is a variant spelling. However, the way to go is giddyap.
Mr. Rewrite had to break out his thick and expensive Webster's New World College Dictionary to corral this one. His paperback version is just a tenderfoot, it seems, as it doesn't list the word.
Because AP follows the New World on matters not covered by its stylebook, Mr. Rewrite has no problem recommending giddyap.
Two dictionaries say the term comes from the phrase "get up." Every reference Mr. Rewrite can find lists giddap as another variant.
So why is it -ap and not -up? Mr. Rewrite's research skills failed him on this question. Any ideas, Community of Concern?
By the way, Westerns in reference to the genre of movies and novels is AP style. So is corralling with two Ls in the headline.
Now Mr. Rewrite has to figure out whether it's yee-haw or yeehaw...
NOTE: Mr. Rewrite had to correct this post because the phrase is trusty mount, not trusted mount.