impertinent

If someone's rude without being openly nasty, like a kid in the back row of class quietly heckling his teacher, you can call him impertinent.

Impertinent originally meant just what it sounds like, "not pertinent, irrelevant," but it also came to mean "inappropriate, out of place" and therefore "intrusive, presumptuous; behaving without proper respect; insolent." It still carries a condescending air, so it's best used of or to a child being snippy to a grownup: "Don't be impertinent!" The stress is on the second syllable: im-PERT-inent.