Atwain

from The Collaborative International Dictionary of English v.0.48

Atwain \A*twain"\, adv. [OE. atwaine, atwinne; pref. a- +
   twain.]
   In twain; asunder. [Obs. or Poetic] "Cuts atwain the knots."
   --Tennyson.
   [1913 Webster]