Modifying a noun of action: performed with great force, exertion, or persistence; involving great labour or effort; vehement, vigorous, violent…

Of action or effort: Vigorous, energetic; now with stronger sense, persistently and ardently laborious. Of conditions, periods, etc.: Characterized…

Of labour, occupation, or work: requiring energy or strength; difficult, rigorous, arduous. See also rough work, n.

By transference to the activity required for the task: Strenuous, energetic, laborious.

That requires considerable effort; severe; laborious, toilsome.

Exhibiting, full of, or requiring effort.

Characterized by great effort, urgency, or a determination to succeed. Often with negative connotations.