The story
From cotton-headed + ninny (“fool”) + muggins (“a gullible person”); implied to be part of the Christmas elf dialect from the North Pole; originally from the Christmas film Elf (2003).
Etymology from Wiktionary, CC BY-SA 4.0.
cotton-headed ninny-muggins
Today it means: A silly fool or simpleton, used as a whimsical insult. Full definition →