by-blow

noun2

1. •
   A blow struck to the side or from the side, as in swordplay; a secondary or incidental strike of any sort.

   • Either commander took speedy advantage of it—Hopton to make a swift diversion into Sussex and capture Arundel Castle (which was but a by-blow, for in a few weeks he had lost it again). — 1910
2. •
   (slang) An illegitimate child; a child of an unknown or unmarried father.

   • Yes, Billy was a foundling, a presumable bye-blow, and, evidently, no ignoble one. — 1892