The feeling
Baggot Brain is the joke-name given to the state that overtakes X and Z when forgotten or apparently new historical figures begin accumulating around them. It starts with the real King Baggot and Tom Magee, then intensifies through Mumfry and Leopold zu Vingt.
The important feature is not ordinary forgetting. The characters are distressed because their identities depend partly on knowing useless cultural history. When an object seems exactly like something they would have known, absence from memory begins to feel like evidence about reality rather than evidence about the limits of expertise.
From joke to engine
The phrase appears just as Leopold zu Vingt's online biography seems to acquire new detail. “Baggot Brain” therefore names a feedback loop: look something up → find more evidence → trust memory less → look harder → find still more evidence. The archive becomes an anxiety amplifier.
The next work, INSTRUCTIONS NEVER TO DESTROY VIEW OR DUPLICATE, externalizes the condition. Instead of wondering whether King Baggot has somehow appeared in history, A and B physically remove his star and descend into the fiction beneath it.
See also Patterning Instinct / Apophenia, Memory & Forgetting and Documentary Instability.