The Collaborative International Dictionary of English v.0.48:

Tantivy \Tan*tiv"y\, adv. [Said to be from the note of a hunting
   horn.]
   Swiftly; speedily; rapidly; -- a fox-hunting term; as, to
   ride tantivy.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Tantivy \Tan*tiv"y\, n.
   A rapid, violent gallop; an impetuous rush. --Cleverland.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Tantivy \Tan*tiv"y\, v. i.
   To go away in haste. [Colloq.]
   [1913 Webster]