• Catalan abstrús
• German abstrus
• Italian astruso
• Middle French abstruse (modern French abstrus, abstruse
• Portuguese abstruso
• Spanish abstruso

• (RP) IPA: /əbˈstɹuːs/
• (America, Canada) IPA: /əbˈstɹus/, /æb-/
• (Scotland, Northern Ireland) IPA: /əb.ˈstɹʉs/

abstruse (comparative abstruser, superlative abstrusest) (formal)

1. Difficult to comprehend or understand; obscure. [from mid 16th c.]

   Synonyms: abstrusive, arcane, cryptic, esoteric, recondite, Thesaurus:incomprehensible

   Antonyms: clear, obvious, understandable, Thesaurus:comprehensible

   • 1671, John Milton, “Samson Agonistes, […].”, in Paradise Regain'd. A Poem. In IV Books. To which is Added, Samson Agonistes, London: […] J[ohn] M[acock] for John Starkey […], →OCLC ↗, page 65 ↗, line 75:

     Be leſs abſtruſe, my riddling days are paſt.

   • 1729, John Machin, “The Laws of the Moon’s Motion According to Gravity”, in Isaac Newton, translated by Andrew Motte, The Mathematical Principles of Natural Philosophy. […] , volume II, London: […] Benjamin Motte, […], →OCLC ↗, page 4 ↗:

     Thoſe propoſitions relating to the Moon's motion, which are demonſtrated in the Principia [by Isaac Newton], do generally depend on calculations very intricate and abſtruſe, the truth of which is not eaſily examined, even by thoſe that are moſt skilful; […]

   • 1748, [David Hume], “Essay I. Of the Different Species of Philosophy.”, in Philosophical Essays Concerning Human Understanding, London: […] A[ndrew] Millar, […], →OCLC ↗, page 3 ↗:

     'Tis certain, that the eaſy and obvious Philoſophy vvill always, vvith the Generality of Mankind, have the Preference to the accurate and abſtruſe; and by many vvill be recommended, not only as more agreeable, but more uſeful than the other.

   • 1788, Publius [pseudonym; Alexander Hamilton], “Number XXXI. The Same Subject Continued [Concerning Taxation].”, in The Federalist: A Collection of Essays, Written in Favour of the New Constitution, […] , volume I, New York, N.Y.: […] J. and A. M‘Lean, […], →OCLC ↗, page 191 ↗:

     The objects of geometrical enquiry are ſo intirely abſracted from thoſe purſuits vvhich ſtir up and put in motion the unruly paſſions of the human heart, that mankind vvithout difficulty adopt not only the more ſimple theorems of the ſcience, but even thoſe abſtruſe paradoxes, vvhich hovvever they may appear ſuſceptible of demonſtration, are at variance vvith the natural conceptions vvhich the mind, vvithout the aid of philoſophy, vvould be led to entertain upon the ſubject.

   • 1816 June – 1831 October 30, Mary W[ollstonecraft] Shelley, chapter IV, in Frankenstein: Or, The Modern Prometheus (Standard Novels; IX), 3rd edition, London: Henry Colburn and Richard Bentley, […], →OCLC ↗, [https://babel.hathitrust.org/cgi/pt?id=emu.000011593191;view=1up;seq=54 page 36]:

     In a thousand ways he smoothed for me the path of knowledge, and made the most abstruse inquiries clear and facile to my apprehension.

2. (obsolete) Concealed or hidden; secret. [late 16th – mid 18th c.]

   Synonyms: abstrused, clandestine, surreptitious, Thesaurus:covert, Thesaurus:hidden

   Antonyms: open, patent, unconcealed, unhidden

   • 1612, [Miguel de Cervantes], Thomas Shelton, transl., “Which Speakes of that which after Befell in the Inne; and of Sundry Other Things Worthy to be Knowne”, in The History of the Valorous and Wittie Knight-errant Don-Quixote of the Mancha. […], London: […] William Stansby, for Ed[ward] Blount and W. Barret, →OCLC ↗, part 4, page 500 ↗:

     O vvho is he that could carrie nevves to our olde father, that thou vvert but aliue, although thou vvert hidden in the moſt abſtruſe dungeons of Barbarie; for his riches, my brothers and mine vvould fetch thee from thence.

   • 1648, Joseph Beaumont, “Canto XVII. The Mortification.”, in Psyche: Or Loves Mysterie, […], London: […] George Boddington, […], published 1651, →OCLC ↗, stanza 182, page 328 ↗, column 2:

     [T]he abſtruſeſt Things / VVhich in the Mindes dark Temper neſtling ly, / By you expoſed are to every Eye.

   • 1667, John Milton, “Book V ↗”, in Paradise Lost. […], London: […] [Samuel Simmons], and are to be sold by Peter Parker […]; [a]nd by Robert Boulter […]; [a]nd Matthias Walker, […], →OCLC ↗; republished as Paradise Lost in Ten Books: […], London: Basil Montagu Pickering […], 1873, →OCLC ↗, 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; […]

• abstrude (obsolete)
• abstrused (adjective, obsolete)
• abstrusion (archaic)
• abstrusive
• abstrusively
• retruse (obsolete)

• French: abstrus, abscons
• German: abstrus, unfassbar
• Italian: astruso, ermetico
• Portuguese: abstruso
• Russian: мудрёный
• Spanish: abstruso