by-blow

noun by-blows · byblow · bye-blow

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).
2. slang 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.’