(noun) Literally “cuckoo’s child”: A child who is not raised by their biological father, but is raised by another man who (wrongly) assumes he is the biological father.
https://de.wikipedia.org/wiki/Kuckuckskind This term comes from the cuckoo bird's tendency to find a nest from another species of bird that already has eggs in it, remove the other bird's eggs and lay their own there, letting the other bird species raise their young. I've seen "mailman's child" or "milkman's child" as possible English translations, but have never heard those used in everyday speech. The wiki article above only links to an English article for "misattributed paternity," but to no other language. So whenever a mother tells her partner he's the father of her kid, and he's actually not, but he ends up raising the kid anyway, that kid is a Kuckuckskind.
Danish: Gøgeunge
Being the adopted one, the kids saw John as the cuckoo in the nest, as the mongoose in their viper den.
“Kakukkfióka” in Hungarian means the same thing literally and figuratively.