This ain’t it, Chief is a slang phrase, used for disillusionment of posters about their own posts, that they consider cool, but are, in fact, absolute trash.
The term is related to the expression “Imma keep it real with you chief” that is similarly friendly but mildly insulting.
The expression started to appear in the summer days of 2018, on social media sites, as well as chatrooms, becoming the new ignition for arguments.
By July, its entry was already added to Urban Dictionary, with another one coming roughly a year later.
It started to appear in 4chan and Reddit threads, used to mock the previous poster and their opinion.
The phrase had also spread onto Twitter, where people finally started to put others into their places and start arguments, others can watch, while munching popcorn and laughing their asses off.
It is still continuing to be used, and can be encountered on basically all social media sites, from Facebook to Instagram and Reddit.