Definition: A fool or a silly person.
Analysis: The word "fool," unless you're Mr. T, is sometimes woefully inadequate to express the stupidity of the person you're talking about. So use Ninnyhammer. Or at least NINNY.

Alternative: The English language is chockful of colorful words meaning stupid person, such as: DUMMKOPF, IGNORAMUS, JOBBERNOWL, GOWK, and WITLING.

For mental retardation, eschew the ubiquitous 'tard - rather, use AMENTIA (extreme mental retardation because of inadequate brain tissue), CRETINISM (mental retardation associated with dwarfism, caused by the deficiency of a thyroid hormone, a person with cretinism is a CRETIN), and MORONITY (used to mean mild retardation of having a mental age of 7 to 12 years, now it's an obsolete term though we still use the word moron).

when you've done something so cringe you can't stop replaying it in your head and it stops you from getting on with your every day life