All 3 Uses

formidable
in
The Red-headed League
(Auto-generated)

The swing of his nature took him from extreme languor to devouring energy; and, as I knew well, he was never so truly formidable as when, for days on end, he had been lounging in his armchair amid his improvisations and his black-letter editions.†
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formidable = intimidating or impressive
I had the hint from Holmes that this smooth-faced pawnbroker's assistant was a formidable man—a man who might play a deep game.†
This also was opened, and led down a flight of winding stone steps, which terminated at another formidable gate.†

Definitions:

(1) (formidable)
intimidating or impressive -- arousing fear or admiration due to impressiveness or challenge
(2) (meaning too rare to warrant focus)