Meaning

originWhoa (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!”).

Etymology from Wiktionary, CC BY-SA 3.0