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

• yours truly
• yourself
• yourselves
• yous
• youse
• youth
• youth crusade
• youth culture
• youth gang
• youth hostel
• youth movement
• youth subculture
• youth-on-age
• youthful
• youthfully
• youthfulness
• yowl
• yowler
• Ypres
• Yquem
• yr
• ytterbite
• ytterbium
• yttria
• yttric