adverb, adjective, interjection or noun
Adv: at full gallop; headlong.
Noun: a hunting-cry.
Adj: swift, speedy.
Int: the sound of the horn or of horses' hooves.
Imitation/description - either of a hunting-horn or horses' hooves. Imagine a horse running at gallop, and say 'tantivy' in time, There you go!
As Mark ran tantivy towards the fence, he wondered idly about the source of the word 'tantivy.' Its remarkable range of meanings seemed all the more remarkable in that no one really knew the origins of this peculiar, but mellifluous word. He had to give pause to his thoughts for a moment as the tantivy sound of the bull's hooves behind him got louder and he unleashed a burst of speed and a screaming tantivy shout, running - horns lowered - tantivy towards the fleeing boy.