expression, noun
remaining a mystery; ending without understanding; never finding out
Describes a situation where something remains unknown or unresolved, often with a nuance of regret or frustration. Commonly used in the pattern 〜ずじまいだ/だった.
結局、彼の本当の気持ちは分からずじまいだった。
In the end, his true feelings remained a mystery.
あの事件の真相は分からずじまいで終わった。
The truth of that incident ended without ever being known.
A grammatical pattern meaning 'ended up not doing', attached to the negative stem of a verb. 分からずじまい is a specific lexicalized instance of this pattern with 分かる.
From 分かる (to understand) in its negative stem 分からず + じまい (ending without doing). The じまい part is a suffix derived from 仕舞い (ending, finishing).