Recently I was reading on this forum website about the difference between 子供 and 子ども, and one of the comments in response was:
I feel like it’s somehow related to 知らず and the --ず(に) grammar.
Based on the context, I have the general impression that this means “oh, I didn’t know that!”, but I’ve never seen anything like that, and was wondering if anyone knew what it means and why.
Well, it looks like ざる, which is the “verb” form of ず. It’s not used in modern Japanese except in some very formal settings/ set phrases, such as 言わざるを得ない.
Then put in the past tense, which does make “I didn’t know that”.
I guess the person was trying on purpose to sound dated?
They list しらざった there among the examples. I’m pretty sure you can find other dialects that use this. Internet slang usually is a combination of various dialects, abbreviations, and standard Japanese so idk , it’s hard to follow message boards for that reason.