A blow struck to the side or from the side, as in swordplay; a secondary or incidental strike of any sort.
(slang) An illegitimate child; a child of an unknown or unmarried father.
c. 1892, Herman Melville, "Billy Budd, Foretopman" (novella), in Herman Melville: Selected Tales and Poems, Holt, Rinehart & Winston (1950), p. 298,
Yes, Billy was a foundling, a presumable bye-blow, and, evidently, no ignoble one.