adj. 25B6; adjective OBSCURE, arcane, esoteric, little known, recherché, rarefied, recondite, difficult, hard, puzzling, perplexing, cryptic, Delphic, complex, complicated, involved, over/above one's head, incomprehensible, unfathomable, impenetrable, mysterious.
ABSTRUSE
Meaning of ABSTRUSE in English
Concise Oxford thesaurus English vocabulary. Краткий оксфордский словарь английского языка тезаурус. 2012