The Collaborative International Dictionary of English v.0.48:

Inbreed \In*breed"\, v. t. [imp. & p. p. Inbred; p. pr. & vb.
   n. Inbreeding.] [Cf. Imbreed.]
   [1913 Webster]
   1. To produce or generate within. --Bp. Reynolds.
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            To inbreed and cherish . . . the seeds of virtue.
                                                  --Milton.
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   2. To breed in and in. See under Breed, v. i.
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WordNet (r) 3.0 (2006):

inbreeding
    n 1: the act of mating closely related individuals