‘Nincompoop’ is casual and often humorous. Not suited for formal situations. My brother acts like a complete nincompoop during family gatherings.
Pro Tip 2/3
Mild Insult
Expresses annoyance or playful teasing, but isn't a harsh insult. When you mixed up the files, you were being such a nincompoop!
Pro Tip 3/3
Archaic Awareness
Be aware it's an old-fashioned term and might not be widely understood by young people. Grandpa called me a nincompoop; it took a while to understand what he meant.