abstruse

• Meaning  -  obscure; profound; difficult to understand

• Synonyms -  abstract, esoteric, difficult, obsolete, recondite.

• Antonyms -  clear, easy, lucid, simple, concrete.

• Usage -  Abstruse subjects are hard to understand.

• Remember -  It contains "ab....use". ab generally stands for negative sense and so 'ab+ use' can be taken as difficult to use. A word will be difficult to use if it is difficult to understand.