WordWeb Online

1. Characterized by a lightly pert and exuberant quality
   "a certain impertinent gaiety and ease of manner"
   - irreverent, pert, saucy
    
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, sassy [N. Amer], wise, lippy

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