Advanced, abstruse, difficult to comprehend; (also) †difficult to perform, arduous (obsolete). In later use frequently with nouns denoting a…

Of a task: Heavy, arduous, difficult. Obsolete.

In occasional figurative uses. (Very common in Milton). Of an aim, an undertaking, etc.: Arduous, full of difficulty, ambitious.

Hard to accomplish or achieve; requiring strong effort; difficult, laborious, severe.

That breaks or is liable to break a leg or legs. Also in extended use: arduous; treacherous.

figurative. Difficult, complex, or challenging; hard to solve, overcome, or deal with. Cf. knotty, adj. 2.

slang (originally and chiefly U.S.). figurative. Arduous, punishing; (also) emasculating. Cf. ball-breaking, adj.