The Collaborative International Dictionary of English v.0.48:

Swithe \Swithe\, adv. [AS. sw[imac]?e strongly, violently.]
   Instantly; quickly; speedily; rapidly. [Obs.]
   [1913 Webster]

         That thou doest, do thou swithe.         --Wyclif (John
                                                  xiii. 27).
   [1913 Webster]