difficult to penetrate; incomprehensible to one of ordinary understanding or knowledge; "the professor's lectures were so abstruse that students tended to avoid them"; "a deep metaphysical theory"; "some recondite problem in historiography"
requiring more than usually expected or thought due; especially great patience and effort and skill; "found the job very demanding"; "a baby can be so demanding"
possessing or exerting or displaying energy; "an energetic fund raiser for the college"; "an energetic group of hikers"; "it caused an energetic chemical reaction"
extremely hot; "the fervent heat...merely communicated a genial warmth to their half-torpid systems"- Nathaniel Hawthorne; "set out...when the fervid heat subsides"- Frances Trollope
inspiring fear; "the formidable prospect of major surgery"; "a tougher and more redoubtable adversary than the heel-clicking, jackbooted fanatic"- G.H.Johnston; "something unnerving and prisonlike about high grey wall"
extremely impressive in strength or excellence; "a formidable opponent"; "the challenge was formidable"; "had a formidable array of compositions to his credit"; "the formidable army of brains at the Prime Minister's disposal"
marked by unjust severity or arbitrary behavior; "the oppressive government"; "oppressive laws"; "a tyrannical parent"; "tyrannous disregard of human rights"
weighing heavily on the senses or spirit; "the atmosphere was oppressive"; "oppressive sorrows"
characterized by care and perseverance in carrying out tasks; "a diligent detective investigates all clues"; "a diligent search of the files"
quietly and steadily persevering especially in detail or exactness; "a diligent (or patient) worker"; "with persevering (or patient) industry she revived the failing business"
not easy; requiring great physical or mental effort to accomplish or comprehend or endure; "a difficult task"; "nesting places on the cliffs are difficult of access"; "difficult times"; "a difficult child"; "found himself in a difficult situation"; "why is it so hard for you to keep a secret?"
requiring much effort and trouble; "the mountain climb was long, steep, and difficult"
having or showing determination and energetic pursuit of your ends; "an aggressive businessman"; "an aggressive basketball player"; "he was aggressive and imperious; positive in his convictions"; "aggressive drivers"
characteristic of an enemy or one eager to fight; "aggressive acts against another country"; "a belligerent tone"
having or showing great excitement and interest; "enthusiastic crowds filled the streets"; "an enthusiastic response"; "was enthusiastic about taking ballet lessons"