KanjiJLPT N1
So-And-So

so-and-so, one, a certain, that person

Mnemonic

Someone carved a name into a tree, but sweet sap oozed over it, leaving only a sticky smear. Now nobody can read whose tree it is. 'It belongs to So-And-So,' people say with a shrug. The sweet residue permanently hides the identity of that mysterious so-and-so.
Additional thoughts
Picture the tree sap (sweet) actively erasing identity — sweetness = anonymity. The obscured carving perfectly mirrors the idea of an unnamed, unspecified person.
Quick recall
Sweet sap on a tree erases a carved name, leaving only So-And-So.

Details

The keyword for 某 is so-and-so. This kanji refers to a certain unspecified person, place, or thing, functioning much like the English expressions "so-and-so," "such-and-such," or "a certain." It is used when the speaker either does not know or deliberately chooses not to reveal the specific identity of what is being discussed. In formal or legal contexts, it often serves as a placeholder for an unnamed individual or entity.
On'yomi
ぼう
Kun'yomi
それがし、なにがし