1. Inspiring fear
   "a tougher and more redoubtable adversary than the heel-clicking, jackbooted fanatic"
   - formidable, unnerving, doubtable [archaic] 
2. Worthy of respect or honour
   "born of a redoubtable family"

See also: alarming, reputable

• gray market
• gray matter
• gray mullet
• gray partridge
• gray polypody
• gray poplar
• gray sage
• gray sea eagle
• gray skate
• gray snapper
• gray sole
• gray substance
• gray whale
• gray willow
• gray wolf
• gray-black
• gray-blue
• gray-brown
• gray-green
• gray-haired
• gray-headed
• gray-leaf pine
• gray-pink
• gray-white
• grayback