The word NANNICOCK is a rare and obselete word but was meant to define a nanny and a pillicock which just means that is if describing a young woman.
The name comes from a time when the rich used to box up gifts to give to the poor. Boxing Day was traditionally a day off for servants, and the day when they received a special Christmas box from their masters. The servants would also go home on Boxing Day to give Christmas boxes to their families.
The phrase ''you're a caution'' is a colloquialism (informal phrase), and may have different meanings in different parts of the country. It usually means you are funny, clever or different in an interestingly way. It's usually used in a positive, complimentary or fun way.