This word is acceptable for play in the US & UK dictionaries that are being used in the following games:

• Lexulous
• Wordscraper
• Wordosaur

adj. Arousing fear, dread, or alarm: the formidable prospect of major surgery.

adj. Inspiring awe, admiration, or wonder: "Though a true hero, he was also a thoroughgoing bureaucrat and politician, a formidable combination” ( Mario Puzo).

adj. Difficult to undertake, surmount, or defeat: a formidable challenge; a formidable opponent.

adj. causing fear, dread, awe or admiration as a result of size, strength, or some other impressive quality; commanding respect

adj. difficult to defeat or overcome

adj. Exciting fear or apprehension; impressing dread; adapted to excite fear and deter from approach, encounter, or undertaking; alarming.

Exciting or fitted to excite fear or apprehension; hard to deal with; difficult to overcome, perform, or the like: applied to persons or things possessing such strength, power, or capability, or presenting such obstacles to action or progress, as to discourage effort or inspire dread of failure.

Synonyms Deterrent, discouraging, fearful, appalling, redoubtable.

adj. extremely impressive in strength or excellence

adj. inspiring fear

"From this latter practice arose their name — CONDOTTIERI; a term formidable all over Italy, for a period, which concluded in the earlier part of the seventeenth century, but of which it is not so easy to ascertain the commencement."

impressive    alarming   

tremendous    frightful    shocking    terrible    terrific    fearful    horrible    dreadful    alarming    redoubtable   

biddable   

terrible    powerful    hostile    impressive    considerable