It's a corruption of the French "Ty a hilaut" which means "The game's afoot!" and is used to advise the Hunt that the hounds have a scent or sight of a fox. The etiquette of fox hunting is French, handed down from written rules by Count Gaston De Foix in the 14th century. This phrase, along with "Illoequs!" which became "Yoiks!" have corrupted over the years. They are popular in the south of England because that area has always enjoyed an active tradition of fox hunting, so the phrases, and corruptions, have been handed down over the generations.