love child

noun2

1. •
   (euphemistic) A child born as a result of a romantic liaison between parents not married to one another; an illegitimate child.

   • Two thousand a-year without debt or drawback — except for the little love-child, indeed; aye, I had forgot her; but she may be 'prenticed out at a small cost, and then what does it signify? — 1811
2. •
   Something that includes influences of multiple predecessors.

   • The grunge genre was the love child of punk and metal.