formidable (adj.)

mid-15c., "causing fear," from Old French formidable (15c.), from Latin formidabilis "causing fear, terrible," from formidare "to fear," from formido "fearfulness, fear, terror, dread." Sense has softened somewhat over time, in the direction of "so great (in strength, size, etc.) as to discourage effort." Related: Formidably.

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Definitions of formidable from WordNet

formidable (adj.)
extremely impressive in strength or excellence;
the challenge was formidable
had a formidable array of compositions to his credit
the formidable army of brains at the Prime Minister's disposal
a formidable opponent
formidable (adj.)
inspiring fear; "a tougher and more redoubtable adversary than the heel-clicking, jackbooted fanatic"- G.H.Johnston;
the formidable prospect of major surgery
Synonyms: redoubtable / unnerving