from The American Heritage® Dictionary of the English Language, 4th Edition
- interj. Used as a command to stop, as to a horse.
from Wiktionary, Creative Commons Attribution/Share-Alike License
- interj. stop (especially when commanding a horse or imitative thereof); calm down; slow down.
- interj. An expression of surprise.
from the GNU version of the Collaborative International Dictionary of English
- interj. Stop; stand; hold. See ho, 2.
from The Century Dictionary and Cyclopedia
- Stop! stand still!
Whoa (c. 1843) is a variant of woa (c. 1840), itself a variant of wo (c. 1787), from who (c. 1450), ultimately from Middle English ho, hoo (interjection), probably from Old Norse hó! (interjection, also, a shepherd's call). Compare German ho, Old French ho ! ("hold!, halt!"). (Wiktionary)