Abstruse

From French abstrus

SOED5|page=10

or its source, Latin abstrÅ«sus ("hidden, concealed"), the perfect passive participle of abstrÅ«dÅ ("conceal, to push away")

MW3 1976|page=8

, itself from ab, abs ("away") + trÅ«dÅ ("thrust, push").

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Cognate with German abstrus.

Full definition of abstruse

(obsolete) Concealed or hidden out of the way; secret. Attested from the late 16^th century until the mid 18^th century.

1612, Thomas Shelton (translator), Miguel de Cervantes (Spanish author), The History of the Valorous and Wittie Knight-Errant Don-Quixote of the Mancha, Part 4, Chapter 15, page 500:O who is he that could carrie newes to our olde father, that thou wert but aliue, although thou wert hidden in the most abstruse dungeons of Barbarie; for his riches, my brothers and mine would fetch thee from thence.
1667, Milton, Paradise Lost:The eternal eye whose sight discerns abstrusest thoughts.

Difficult to comprehend or understand; recondite; obscure; esoteric. First attested in the late 16^th century.

1548, Bishop John Hooper, A Declaration of the Ten Holy Comaundementes of Almygthye God, Chapter 17 Curiosity, Page 218:...at the end of his cogitacions, fyndithe more abstruse, and doutfull obiections then at the beginning...
1748, David Hume, Enquiries concerning the human understanding and concerning the principles of moral. London: Oxford University Press, 1973. Â§ 13.It is certain that the easy and obvious philosophy will always, with the generality of mankind, have the preference above the accurate and abstruse;
1855, Henry Hart Milman, History of Latin Christianity:Profound and abstruse topics.

More abstruse and most abstruse are the preferred forms over abstruser and abstrusest.