anonymous person; a nobody
Refers to someone whose name is unknown or intentionally withheld. Can carry a dismissive tone when meaning 'a nobody'.
The letter arrived from an anonymous person.
彼はただの無名氏に過ぎない。
He is nothing more than a nobody.
名無し is a more casual or colloquial term for 'nameless' or 'anonymous', often used in online contexts.
Compound of 無名 (mumei, 'nameless, anonymous') and 氏 (shi, 'person, Mr./Ms.'). The exact historical derivation is uncertain, but the combination straightforwardly means 'nameless person'.