1. (a.) Difficult to understand; abstract; esoteric; as abstruse lectures\.

2. (a.) Remote from apprehension; difficult to be comprehended or understood; recondite; as, abstruse learning.

Herculean abstract arcane arduous beclouded blind brutal buried civilized close clouded complex complicated concealed covered covert critical cultivated cultured deep delicate demanding difficile difficult eclipsed educated encyclopedic erudite esoteric exacting formidable hairy hard hard-earned hard-fought heavy hermetic hid hidden hypothetical ideal in a cloud in a fog in eclipse in purdah in the wings incommunicado intricate jawbreaking knotted knotty laborious latent learned lettered literate mean mysterious no picnic not easy obfuscated obscure obscured occult operose pansophic polyhistoric polymath polymathic profound recondite rigorous rough rugged scholarly scholastic secluded secluse secret sequestered set with thorns severe spiny steep strenuous studious thorny ticklish toilsome tough transcendental tricky under an eclipse under cover under house arrest under wraps underground unknown uphill wicked wise wrapped in clouds