illegitimate child; secret love child
Refers to a child born out of wedlock or kept hidden from public knowledge, often with a nuance of secrecy or scandal.
There is a rumor that he has a secret love child.
The existence of the politician's illegitimate child came to light.
Compound of 隠し (kakushi, 'hiding') + 子 (ko, 'child'). The exact historical derivation is uncertain, but the word has been used since at least the Edo period.