Giddap

gid·dy·up

 (gĭd′ē-ŭp′) also gid·dy·ap (-ăp′, -ŭp′) or gid·dap (gĭ-dăp′, -dŭp′)
interj.
Used to command a horse to go ahead or go at a faster pace.

[Alteration of get up.]
American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.

giddap

(ɡɪˈdæp) or

giddy-up

interj
an exclamation used to make a horse go faster
[C20: colloquial form of get up]
Collins English Dictionary – Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014

Giddap

Command to a horse to start or to move faster.
1001 Words and Phrases You Never Knew You Didn’t Know by W.R. Runyan Copyright © 2011 by W.R. Runyan
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References in periodicals archive ?
linocut by Hale Woodruff entitled Giddap (1935) showed the threatening
So the farmer called "Giddap" to Old Blue, and they trotted home.
were sitting behind him playing horse, and whipping him to giddap." In their poverty the family "pawned everything at one time or another, including the children's shoes and Marx's coat--which prevented them from going out of doors." It sometimes got so bad that his wife "would cry all night, and Karl would lose his temper: he was trying to write his book on economics." There's a humorous flourish about this final sentence, in which the immediate economic hardships frustrate Marx's attempts to complete Das Kapital, his magisterial study of the economic force in history.
When they got to the gate the light was red, but the policeman ignored it and shouted "giddap" to the horse and drove on through the red light.