noun
owner of a lost item; person who dropped something
Refers to the person who lost or dropped an item, often used in contexts like lost-and-found or returning belongings.
落とし主が見つからないので、財布を交番に届けた。
Since the owner couldn't be found, I turned in the wallet at the police box.
この傘の落とし主をご存じですか。
Do you know who dropped this umbrella?
Compound of 落とし (dropping, from verb 落とす) and 主 (owner, master).