indubious /in-dūˈbi-əs/ (archaic)

1. Not dubious
2. Certain

• letters of administration
• letters of comfort
• letters of credence
• letters of credit
• letters of horning
• letters of indication
• letters of intercommuning
• letters-of-marque
• Letters of peace
• Letters pacifical
• letters patent
• letters requisitory
• letters rogatory
• letter-stamp
• letter-stamps
• letter-weight
• letter-weights
• letter-wood
• letter-writer
• letter-writers
• let the cat out of the bag
• let the grass grow under one's feet
• let the side down
• Lettic
• letting