The Collaborative International Dictionary of English v.0.48:

An \An\, conj. [Shortened fr. and, OE. an., and, sometimes and
   if, in introducing conditional clauses, like Icel. enda if,
   the same word as and. Prob. and was originally pleonastic
   before the conditional clause.]
   If; -- a word used by old English authors. --Shak.
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         Nay, an thou dalliest, then I am thy foe. --B. Jonson.
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   An if, and if; if.
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