Tantivy means “a gallop” as a noun, “swift;rapid” as an adjective, and “at full gallop” as an adverb. It’s also an interjection “used as a hunting cry when the chase is at full speed”.
Webster’s Revised Unabridged Dictionary reports that the word may come “from the note of a hunting horn”. World Wide Words found that some dictionaries “say it imitates the sound of the hooves of galloping horses”, but it’s more likely to come from this:
"the three notes of the huntsman