What is the X in X-mas?
"characterized by great effort," 1590s, from Latin strenuus "active, vigorous, keen." Probably cognate with Greek strenes, strenos "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: Strenuously; strenuousness.