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

• syllabled
• syllabub
• syllabus
• syllepsis
• syllogise
• syllogiser
• syllogism
• syllogist
• syllogistic
• syllogistically
• syllogize
• syllogizer
• sylph
• nonmalignant tumor
• nonmalignant tumour
• nonmandatory
• nonmaterial
• nonmeaningful
• nonmechanical
• nonmechanistic
• nonmedicinal
• nonmember
• nonmetal
• nonmetallic
• nonmetamorphic