is an ateji, which means either _the readings of the individual kanji do not match the reading of the word_ , or _the meanings of the individual kanji do not match the meaning of the word_. In this case, it's the latter - why would an idiot be described as a horse and/or a deer?
As for why these particular kanji were chosen to represent , the etymology is unclear. See this webpage for several theories. <
In this case, the individual kanji meanings were forgone. Other ateji include {} (readings forgone), {} (readings forgone), {} (readings forgone), {} (meanings forgone), etc.