Super cute moment with Husband recently. We were watching Modern Family when he explained to me that he thinks Jay Pritcherd (sp?), played by Ed O'Neil, was actually Al Bundy from Married...With Children (samr actor) after he struck it rich and left his wife, Peg. I grinned at him and told him that's a headcanon. He had no idea what I meant. So I explained to him that an established work such as a TV show or a movie or novel or comic, etc., is called "canon,"
from the Latin meaning "church law," and conversely any background or explanation or conjecture a fan comes up with, is called "headcanon," because it comes from their own head/imagination. It did my fangirl heart good.