noun which may take the genitive case particle 'no'
anonymous; of unknown authorship
Used to describe a work whose author is unknown, such as an anonymous poem, painting, or manuscript.
この絵は作者不明だが、江戸時代のものらしい。
This painting is of unknown authorship, but it seems to be from the Edo period.
作者不明の詩が古い文献に残っている。
An anonymous poem remains in an old document.
匿名 means 'anonymous' in the sense of a person hiding their identity, while 作者不明 refers specifically to a work whose creator is unknown.
Compound of 作者 (author) and 不明 (unknown). The exact historical derivation is uncertain, but the term is straightforwardly formed from these two Sino-Japanese elements.