1
general::
1. Also, full of the devil. Mischievous, naughty. For example, The youngsters were full of it today, giving the teacher a hard time, or Bill is full of the devil, hiding his roommate's clothes and teasing him mercilessly. 2. Talking nonsense, as in He claims to have fixed the dock, but I think he's full of it. This usage is a euphemism for ruder idioms like FULL OF CRAP.
American Heritage Idioms