The horse's ass that thinks they're the smartest horse's ass in the barn..

The smartest person in a very small limited group or space, for example a stable.

unmitigated moron

First heard when Donald Trump described himself as a stable genius, implying that he was a very bright individual. Trump who had and continues to demonstrate he is anything but a genius, means the term takes on quite a different meaning. Instead, it is used when describing a person who has demonstrated an unusually low intellect. When used to describe a person or a person's behavior, it references an extreme idiot who is totally aware of how poorly he is perceived by all those around him.

The Definition given by the Oxford Dictionary is pretty: "the person that has at least half the IQ presented byDonald J. Trump".