muggins
An Oxford Languages dictionary says:
muggins: (INFORMAL•BRITISH) a foolish and gullible person (often used humorously to refer to oneself).
I heard it used in the form of a convention panelist saying something like “muggins over here” to refer to themselves, indicating that they had done something that they later realized was foolish.
My dictionary (Merriam-Webster) adds that the word can also mean “a provision in many games played in England that if a player fails to record an earned score promptly his opponent may say muggins and claim that score—used especially in cribbage and dominoes,” or “any of various card games in which a score overlooked may be claimed by saying muggins or in which the player with the worst score is called muggins.”