abstruse: [16] It is not clear whether English borrowed abstruse from French abstrus(e) or directly from Latin abstrūsus, but the ultimate source is the Latin form. It is the past participle of the verb abstrūdere, literally ‘thrust’ (trūdere) ‘away’ (ab). (Trūdere contributed other derivatives to English, including extrude and intrude, and it is related to threat.) The original, literal meaning of abstruse was ‘concealed’, but the metaphorical ‘obscure’ is just as old in English.
