By the look and sound of it, cockamamy (also spelled "cockamamie") seems like an arbitrarily coined nonsense word-but a reasonable explanation for its origin exists. Supposedly, cockamamy is an altered form of the term decalcomania, which denotes a process of transferring pictures and designs from specially prepared paper to surfaces such as glass or porcelain. The word decalcomaniacomes from the combination of French décalquer, meaning "to copy by tracing," and -manie, meaning "mania." In the 1940s, painted strips of paper with images capable of being transferred to the skin were called "decals" or "cockamanies." They were naturally regarded by many as silly novelties. Hence, in time, the variant cockamamie came to be used as an adjective meaning "ridiculous.”

– Merriam-Webster