Probably an elaboration of yippee (“used to express excitement”) and yay (“used to express happiness”). The term was associated with the Western United States in the 19th century, and may have originated from scat syllables in so-called “cowboy songs”—compare “Coma ti yi youpy, youpy ya, youpy ya, / Coma ti yi youpy, youpy ya”, the chorus of “The Old Chisholm Trail” which dates to the 1870s and was first published in 1910. The specific term yippee ki-yay was possibly influenced by the line “yippee yi yo kayah”, from the chorus of the 1936 song “I’m an Old Cowhand (From the Rio Grande)” written by the American singer and songwriter Johnny Mercer (1909–1976) for the film Rhythm on the Range and sung by Bing Crosby (1903–1977). Sense 2 (“used to express defiance against, or to startle, an opponent”) refers to the film Die Hard (1988), in which police detective John McClane (played by Bruce Willis), responding to the taunt “Do you really think you have a chance against us, Mr. Cowboy?” by the terrorist leader Hans Gruber (Alan Rickman) whom he is hunting down, says “Yippee ki-yay, motherfucker.” The line was written by the screenwriter Steven E. de Souza (born 1947), based on the expression “Yippee ki yah, kids” apparently said by the American actor and singer Roy Rogers (1911–1998) who frequently played cowboy roles.

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