By-blow

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!

‘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.

The best hope has been that friends and family would talk, a hope partly realized in this discreet but perceptive memoir by his illegitimate daughter. . . . Little Mary was a by-blow and an inconvenience.