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

• correspondence school
• correspondent
• corresponding
• correspondingly
• corrida
• corridor
• corrie
• corrigenda
• corrigendum
• corrigible
• corrival
• corroborant
• corroborate
• corroborating evidence
• corroboration
• corroborative
• corroboratory
• corroboree
• corrode
• corroded
• Corrodentia
• corrodible
• corroding
• corrosion
• corrosion-resistant