A hypothetical alkaloid or chemical compound (the exact nature is obscure in historical usage).
From Greek apo- 'away from' combined with thesine or an unknown root; this appears to be a rare or obsolete medical/alchemical term with uncertain etymology.
This word is so obscure it barely appears in any modern dictionaries—it's a ghost word, possibly created by a 17th-century apothecary or copied by mistake and never verified, showing how language contains linguistic fossils!
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