"characterized by great effort," mid-15c. (implied in strenuously), from Latin strenuus "active, brisk, quick, nimble, prompt, vigorous, keen." Probably cognate with Greek strēnes, strēnos "keen, strong," strenos "arrogance, eager desire," Old English stierne "hard, severe, keen" (see stern (adj.)). Mocked by Ben Jonson as a pedantic neologism in "Poetaster" (1601). Sense of "requiring much energy" is first recorded 1670s. Related: Strenuousness; strenuosity.