1) Showing some cheek 2) Youthful prank in a car 
Mooning is the art of displaying one`s bare buttocks by removing clothing, e.g., by lowering the backside of one`s trousers and underpants, usually bending over, whether also exposing the genitals or not. Mooning is used mostly in the English-speaking world to express protest, scorn, disrespect, or provocation but can also simply be done for sho.....
• (p. pr. & vb. n.) of Moon
An old North-country term for spangling.