deep-seated doubt; unresolvable doubt
Refers to a lingering, often fundamental doubt that cannot be easily dispelled. Literary and somewhat formal.
彼の説明を聞いても、心の奥に疑団が残った。
Even after hearing his explanation, a deep-seated doubt remained in my heart.
Compound of 疑 (doubt) and 団 (group, mass), suggesting a mass or cluster of doubts. The exact historical derivation is uncertain.