The Collaborative International Dictionary of English v.0.48:

By-blow \By"-blow`\, n.
   1. A side or incidental blow; an accidental blow.
      [1913 Webster]

            With their by-blows they did split the very stones
            in pieces.                            --Bunyan.
      [1913 Webster]

   2. An illegitimate child; a bastard.
      [1913 Webster]

            The Aga speedily . . . brought her [his disgraced
            slave] to court, together with her pretty by-blow,
            the present Padre Ottomano.           --Evelyn.
      [1913 Webster]

WordNet (r) 3.0 (2006):

by-blow
    n 1: the illegitimate offspring of unmarried parents [syn:
         bastard, by-blow, love child, illegitimate child,
         illegitimate, whoreson]