The Collaborative International Dictionary of English v.0.48:

Erst \Erst\ ([~e]rst), adv. [Orig. superlative of ere; AS.
   [=ae]rest. See Ere.] [Archaic]
   1. First. --Chaucer.
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   2. Previously; before; formerly; heretofore. --Chaucer.
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            Tityrus, with whose style he had erst disclaimed all
            ambition to match his pastoral pipe.  --A. W. Ward.
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   At erst, at first; at the beginning.

   Now at erst, at this present time. --Chaucer.
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WordNet (r) 3.0 (2006):

erst
    adv 1: at a previous time; "at one time he loved her"; "her
           erstwhile writing"; "she was a dancer once"; [syn:
           once, formerly, at one time, erstwhile, erst]