na-adjective, noun which may take the genitive case particle 'no', noun
unknown; unidentified; unspecified
Used in formal or written contexts to indicate that details such as a person's identity, origin, or date are not known or not given. Often appears in reports, news articles, and official documents.
犯人の身元は不詳のままだ。
The identity of the perpetrator remains unknown.
The date is listed as unknown on this document.
An unidentified body was found.
This is an old painting of unknown authorship.
The details of the cause of the accident are unknown.
未詳 and 不詳 are near-identical in meaning and formality, both used in reports to mean 'details unknown', though 未詳 appears more often after author/date fields (e.g. 作者未詳).
判明 means that facts have become clear or been confirmed, the opposite of 不詳's sense that details remain unknown.
不祥 (as in 不祥事) means 'inauspicious/scandalous' and is a completely different word that just happens to share the same reading as 不詳.
不肖 means 'unworthy' (often used self-deprecatingly), another homophone of 不詳 with an unrelated meaning.
unidentified; name unknown
Compound of 不 (not) and 詳 (detailed, clear). The meaning 'unknown' derives from the idea of something not being clearly detailed or specified.