WordWeb Online

1. Lively and humorously cheeky or forward
   "a certain impertinent gaiety and ease of manner";
   - irreverent, pert, saucy [informal], cheeky [Brit, informal]
    
2. Not pertinent to the matter under consideration
   "mentioned several impertinent facts before finally coming to the point";
   - extraneous, immaterial, orthogonal
    
3. Improperly forward or bold
   "impertinent of a child to lecture a grownup";
   - fresh, impudent, overbold, smart, saucy [informal], sassy [N. Amer, informal], wise, lippy [informal]

See also: beside the point, forward, irrelevant, spirited