When someone shows cheekiness, they are a little rude — they talk back, they tease, they say the thing a careful person would keep in — but it is often meant playfully, and people may half-laugh. We use this mostly of children, teenagers, or friends being boldly familiar, not of someone who is truly cruel. It is informal. Cheeky is the adjective (a cheeky grin); cheekiness is the noun, the being like that. Though actually, whether it feels charming or just rude depends on who is speaking and who has to hear it: the same joke can sound sweet from a small child and sharp from a stranger.