1. Open to doubt or suspicion
   - doubtful, dubious, dubitable, in question 
2. [archaic] Inspiring fear
   - formidable, redoubtable, unnerving

See also: alarming, questionable

• unimpeachably
• unimpeded
• unimportance
• unimportant
• unimposing
• unimpregnated
• unimpressed
• unimpressionable
• unimpressive
• unimpressively
• unimprisoned
• unimproved
• unincorporated
• unindustrialised
• unindustrialized
• uninebriated
• uninfected
• uninflected
• uninfluenced
• uninfluential
• uninformative
• uninformatively
• uninformed
• uninhabitable
• uninhabited