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

• John Luther Jones
• John Lyly
• John M. Browning
• John Macleod
• John Major
• John Marquand
• John Marshall
• John Marstan
• John Masefield
• John Maynard Keynes
• John McCormick
• John McGraw
• John Mercer
• John Merven Carrere
• John Mill
• John Millington Synge
• John Milton
• John Milton Cage Jr.
• John Mitchell
• John Moses Browning
• John Muir
• John Napier
• John of Gaunt
• John Orley Allen Tate
• John Osborne