Definition:
An "illegitimate child" is a noun that refers to a child who is born to parents who are not married to each other. In many societies, this term has been used historically to describe children who were born outside of marriage.
In more advanced discussions, you might encounter terms like "birthright," "legitimacy," or "parental rights," which can relate to the legal and social implications of being an illegitimate child.
While there aren't specific idioms or phrasal verbs directly related to "illegitimate child," phrases like "born on the wrong side of the tracks" can sometimes be used metaphorically to describe someone with a challenging background, though it is often more related to socio-economic status than family structure.