The Collaborative International Dictionary of English v.0.48:

Costard \Cos"tard\ (k?s"t?rd), n. [Prob. fr. OF. coste rib,
   side, F. c[^o]te, and meaning orig., a ribbed apple, from the
   ribs or angles on its sides. See Coast.]
   1. An apple, large and round like the head.
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            Some [apples] consist more of air than water . . .;
            others more of water than wind, as your costards and
            pomewaters.                           --Muffett.
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   2. The head; -- used contemptuously.
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            Try whether your costard or my bat be the harder.
                                                  --Shak.
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