I Have a question if I can. Namely, I am translating a book these days, and desperately need an idiom that means that somebody grows up unattended, means nobody takes care of his education and manners.
A really interesting request, there aren’t actually too many idioms for this (surprising I know!)
The example that springs to mind is:
He is a bad apple.
Behind Someone's Back [idiom] - MyEnglishTeacher.eu
[Tweet “A Bad Apple – Someone who has been brought up incorrectly and spoils a group.”]
This is to say that someone has been brought up incorrectly and spoils a group. The expectation is that this individual will not change and will always remain somebody with bad manners.
The example below shows the use of “a bad apple”.
A: I was walking along the road the other day, and that Kieran kid walked right into me!
B: How awful!
A: The worst part was that he didn’t even apologise!
B: He’s a bad apple that one, I know his mother.
A: Well that explains it then!
The phrase comes from apple picking, if you picked a group of good apples and just one bad one, it could spoil the whole lot of them!