the word 'whoa' is an onomatopoeic interjection, originating as a command to make a horse stop or slow down. it is believed to be a variant of 'ho', a much older exclamation used to command attention or halt movement, which itself has germanic origins. the spelling stabilized as 'whoa' in the 19th century, influenced by its drawn-out pronunciation. from its specific equestrian use, the term broadened into general english as an exclamation to express surprise, amazement, or a desire to pause any activity.

this is a simplex word, a direct loan and onomatopoeic command. it does not break into smaller meaningful morphemes within modern english spelling.