WordWeb Online

1. Not conforming to legality, moral law, or social convention
   "improper banking practices";
   - unconventional, unlawful
    
2. Not appropriate for a purpose or occasion
   "said all the improper things";
   - wrong
    
3. Not suitable, right or appropriate
   "slightly improper to dine alone with a married man"; "improper medication"; "improper attire for the golf course"

See also: correctitude, inappropriate, incorrect, indecent, indecorous, indelicate, irregular, out of the way, out-of-the-way, properness, propriety, unbecoming, uncomely, unfit, unseemly, untoward, wrong

Antonym: proper