by-blow

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).

As a by-blow in the course of his relentless campaigns against Louis, Willem gained the three thrones of Britain in 1699 – but what a by-blow this proved!

An illegitimate child; a child of an unknown or unmarried father.

‘Nay,’ said the landlady, ‘if I thought he was a gentleman's son, thof he was a bye-blow, I should behave to him in another guess manner; for many of these bye-blows come to be great men, and, as my poor first husband used to say, never affront any customer that's a gentleman.’

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.