loppy

知らない (shiranai) can be a pretty biting way to say you don’t care about something/someone’s situation, but it’s not vulgar. しらねぇ (shiranee) would be a more vulgar way to say the same thing, that’s probably closer. I don’t know if there’s a metaphorical idiomatic phrase like the other languages in the OP.

For vulgarity in Japanese in general, you should watch this: https://youtu.be/GcjbwkBQeO8

Lol, yeah. Most of his videos are a couple of minutes, but sometimes he wants a little bit more…

Relevant for anyone who cares about this sort of thing: https://youtu.be/GcjbwkBQeO8