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The Collaborative International Dictionary
Atwain

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

Wiktionary
atwain

adv. (context archaic English) In twain; asunder.