abstruse (adj.)

• abstract
• arcane
• arduous
• blind
• brutal
• buried
• civilized
• close
• clouded
• complex
• complicated
• concealed
• covered
• covert
• critical
• cultivated
• cultured
• deep
• delicate
• demanding
• difficult
• educated
• encyclopedic
• erudite
• esoteric
• exacting
• formidable
• hairy
• hard
• hard-fought
• heavy
• hermetic
• hidden
• hypothetical
• ideal
• incommunicado
• intricate
• knotted
• knotty
• laborious
• latent
• learned
• lettered
• literate
• mean
• mysterious
• obscure
• occult
• operose
• profound
• recondite
• rigorous
• rough
• rugged
• scholarly
• scholastic
• secluded
• secret
• sequestered
• severe
• spiny
• steep
• strenuous
• studious
• thorny
• ticklish
• toilsome
• tough
• transcendental
• tricky
• underground
• unknown
• uphill
• wicked
• wise