Which halachah [hint:Choshen Mishpat] applies to "mi sh'aino yodeya"?
(I ran across this last night researching another question)
"שאינו יודע" turns up, according to a search of mine, 28 times in Choshen Mishpat. But I think you may be referring to this one (228:4, in the halachos of "ona'as devarim," not to wrong someone verbally):
אם נשאלה שאלה על דבר חכמה, לא יאמר למי שאינו יודע אותה חכמה מה תשיב בדבר זה.
"If a question is asked about some area of knowledge, you should not ask someone who is not knowledgeable in it, "What would you answer to this?"
I think the question is a bit open-ended. "I'm thinking of a number between one and a thousand ..."
But over in Even HaEzer, we have the very important concept of
מי שאינו יודע בטיב גטין וקידושין, לא יהא לו עסק עמהן
"Don't officiate at Jewish weddings unless you know what you're doing."
If you go to some Jewish history museums, you're likely to find a Kesubah from 100+ years ago listing a location of "Northern America, which is at the edge of the ocean." Rabbi J. David Bleich observes that the rabbi who coined this notation was described by the Aruch L'Ner as אינו יודע בטיב גטין וקידושין.