Idjit is a slang version of the word idiot, originating from the mispronunciation of the original word, which is used to refer to someone with severe mental incapability.
It is especially prevalent in the southern states of the US and in Supernatural.
Southern dialects changed many words through pronunciation and one of these words was “idjit”.
While written examples are rare to encounter in archives, it has been pronounced so for decades in the southern part of the United States.
“Idjit” started to gain more popularity following the success of the TV show Supernatural, in which Bobby, a character originating from the south, was eagerly using “idjit” in moments of stress.
The term was first defined on Urban Dictionary in 2003 and many of the entries contain a GIF of Bobby from Supernatural.