• (RP) IPA: /əbˈstɹuːs/
• (GA) IPA: /æbˈstɹus/, /əb-/

abstruse (comparative abstruser, superlative abstrusest)

1. Difficult to comprehend or understand. [from late 16th c.]

   Synonyms: esoteric, obscure, recondite

   • 1729, John Machin, “The Laws of the Moon’s Motion According to Gravity”, in Isaac Newton; Andrew Motte, transl., The Mathematical Principles of Natural Philosophy. [...] Translated into English […] In Two Volumes, volume II, London: Printed for Benjamin Motte, […], OCLC 723174213 ↗, page 4 ↗:

     Thoſe propoſitions relating to the Moon's motion, which are demonſtrated in the Principia [by {{w

2. (obsolete) Concealed or hidden out of the way; secret. [from late 16th c. until mid 18th c.]
   • 1667, John Milton, “Book V”, in Paradise Lost. A Poem Written in Ten Books, London: Printed [by Samuel Simmons], and are to be sold by Peter Parker […] [a]nd by Robert Boulter […] [a]nd Matthias Walker, […], OCLC 228722708 ↗; republished as Paradise Lost in Ten Books: The Text Exactly Reproduced from the First Edition of 1667: […], London: Basil Montagu Pickering […], 1873, OCLC 230729554 ↗, lines 709–714:

     [...] Mean while th’ Eternal eye, whoſe ſight diſcernes / Abſtruſeſt thoughts, from forth his holy Mount / And from within the golden Lamps that burne / Nightly before him, ſaw without their light / Rebellion riſing, ſaw in whom, how ſpred / Among the ſons of Morn, what multitudes / Were banded to oppoſe his high Decree [...]

• (concealed) clandestine, secret, surreptitious; See also Thesaurus:covert
• (difficult to comprehend) esoteric, obscure, recondite; See also Thesaurus:incomprehensible

• French: abstrus
• Italian: astruso
• Portuguese: abstruso, abstruso
• Spanish: abstruso

• Portuguese: abstruso