If you speak or act cheekily, you are a little rude, but often in a way that makes people smile — you talk as if you are braver, or closer, than the rules say. A child who answers a teacher with a grin and a sharp joke is being cheeky. It is lighter than rude or impolite. British English uses this a lot; American English may say sassy or fresh for a similar feeling. You will hear it about teasing and small boldness, not about a real hurt. Though actually, if an adult uses it with a cold voice, it can cut more than the word itself suggests, so listen to the tone. The adverb is just cheeky + -ly.