A term used in reference to a male who, as a way of showing his affection to a female, protects her very zealously from the advances, real or imagined, of other males, despite the fact that she has no interest in him or even a boyfriend.

This leads to the following other slang terms:
The castle--the girl in question
The king--the girl's boyfriend (as in "the king is in the castle"
To storm the castle--to hit on the girl, not seriously but just to antagonize the porticullis
Also: boiling oil, trebuchet, battering ram, protracted seige, etc. can be used allegorically to describe specific situations that arise in this setup.

When you eat dirt for the first time and leave your family to build a shack in the woods