Definition of "headcanon"

Technically, the social context of Borogravia is such that the actual external difference between a trans man and a woman disguised as a man is practically indiscernible – the technology level means they’re indistinguishable by the bodily modification standard (although now that I think about it, there's no reason Igors couldn't perform sex changes, and that's going in my headcanon now), and there does not appear to be a space in the language for self-identification.

2012, Maggie K, "Let's talk about socks: Gender performativity in Terry Pratchett's Monstrous Regiment", Queer Vertigo 2012 (University of Technology, Sydney), unknown date, page 17

That wouldn't be so bad if it weren't for the fact that they just fill in the missing information with whatever headcanon (one's personal belief of what is true to the universe) they want instead of actually trying to find out more about the character.

2012 June 11, Bea Huff, “Love the character, not the actor”, in Daily Nebraskan, volume 111, number 151, Lincoln: University of Nebraska, page 3

Don't forget if they can write pages and pages and pages of “headcanon” information for an already developed character, probably changing his or her already set personality or even expanding upon it.

2013, Kristal Frenero, "Fifty shades of fan crazy", The Talon Newsmagazine (John A. Ferguson Senior High), Volume 10, Issue 2, February 2013, page 8

From headcanoning him, kicking his legs, and blushing at a compliment to wearing cat ears, Ghost’s menacing persona is being accepted as a cute little quirk to the girlies. In response, much of COD’s male fanbase is accusing this mostly-female demographic of babygirlifiers of sexualizing him.

2022 November 30, Rachel Choi, “The babygirlification of Ghost in COD is threatening the incels”, in The Berkeley Beacon‎