Someone Or Other
You-Know-Who, Something
noun, pronoun, の adjective
The kanji means certain, but when it's used as a standalone word, or after a family name, it means things like someone or other, you-know-who, or something. It's basically a vague way to refer to someone who's name you've forgotten, or you don't want to mention for some reason.
This word uses the kun'yomi of the kanji, and you haven't learned that yet, so here's a mnemonic to help you:
You bump into someone or other, and just as you're wondering what on earth her name is, her nanny gashes (なにがし) her on the leg. It looks like it hurts. Poor you-know-who, whatever her name is. The nanny looks horrified, and is trying to patch her up.
これは、山田某の話である。
This is the story of Yamada someone or other.
たしか、鈴木某とかいう人じゃなかったっけ？
Wasn't it a guy named Suzuki something?
あそこの劇団の某がまた何かやらかしたらしいぞ。
I heard you-know-who from that drama troupe has done it again.