The 3rd cat in any household, completing a kitty cabal, ultimately leading to the death of everyone in your household. A little known ancient designation originating in Egypt in the 1st Kingdom, and was the undoing of more than one Royal Family. The main reason British Royalty has always preferred canine companionship. Rumored to be the most likely cause of death in the case of Cleopatra. However, this was suppressed for centuries by the Meowmiati, a clandestine organization made up of elderly cat ladies, who changed the narrative to blame a serpent, further vilifying the snake.

Briefly, "the realization that each random passerby is living a life as vivid and complex as your own." Originally from the Dictionary of Obscure Sorrows, which has a lot more beautiful neologism definitions like this you might enjoy.