Green’s Dictionary of Slang

a child to wehom you have no blood relationship, i.e. Some Other Cunt’s Kid.

2010		Urban Dict. 8 July 🌐 ‘Jesus dude! Stay away from her, she has a SOCK!’ ‘A what?’ ‘A fuckin’ SOCK! Some Other Cunt's Kid!’.
2021		Betoota-isms 72: SOCK [...] 1. An acronym for Some Other Cunt’s Kid 2. Term used by the friends of a person currently managing children that are not biologically theirs.