1.
true nature, true character, identity
The real nature or identity of something. Almost exclusively used in the negative expression 得体の知れない (of unknown nature, mysterious, suspicious).
Usage
Almost always used in the set phrase 得体の知れない, meaning "of unknown nature," "mysterious," or "unidentifiable." This expression modifies nouns to convey suspicion, eeriness, or the unsettling quality of something whose true identity cannot be determined. The standalone word 得体 is rarely used outside this pattern.