Someone who's misbegotten is born to two parents who aren't legally married. In the past, being misbegotten was quite embarrassing.
The adjective misbegotten, which commonly means "badly planned," used to have another use: a derogatory way to describe someone born "out of wedlock," or to unmarried parents. It's an extremely old fashioned way to use the word, coined in the 1500s, but you may come across it in a book, describing a pitiful, misbegotten heroine for example. It comes from the equally obsolete misbeget, from the literary definition of beget, "give birth," or "parent."