Abstru'se. adj. [abstrusus, Lat. thrust out of sight.]

1. Hidden.

             Th' eternal eye, whose sight discerns
   Abstrusest thoughts, from forth his holy mount,
   And from within the golden lamps that burn
   Nightly before him, saw, without their light,
   Rebellion rising. Milton's Paradise Lost, b. v. l. 712.

2. Difficult, remote from conception or apprehension. It is opposed to obvious and easy.

   So spake our Sire, and, by his count'nance, seem'd
   Entr'ring on studious thoughts abstruse. Parad. Lost, b. viii.

   The motions and figures withing the mouth are abstruse, and not easy to be distinguished, especially those of the tongue, which is moved through the help of many muscles, so easily, and habitually, and variously, that we are scarce able to give a judgment of motions and figures thereby framed. Holder's Elements of Speech.

   No man could give a rule of the greatest beauties, and the knowledge of them was so abstruse, that there was no manner of speaking which could express them. Dryd. Dufresnoy.