The Brief: "This ain't it chief" is a way to respond to something online, essentially saying "no" and shutting it down.

DEEP DIVE
Similar to “weird flex but ok,” the phrase “this ain’t it chief” is used as a clapback for something someone disagrees with. It also works as a response to a fail or s**tpost.
me: i'm good at the new internet slang
friend: weird flex but ok
me: actually i'm proud of my ability to adopt a new method of communication but go off i guess??
friend: that ain't it chief
— the hype (@TheHyyyype) October 2, 2018
Got off the phone with Chief. He said that this ain’t it pic.twitter.com/IByUdojEys
— The Black Sheep VCU (@blacksheep_vcu) October 7, 2018
— mason morgan (@mason_morgan22) October 4, 2018
that’s enough live streaming for today cus this mine craft quality just ain’t it for me chief 😔 pic.twitter.com/3d2o3oqYF8
— 𝒸𝒶𝓉𝒽 (@cocoadiosa) October 6, 2018
“this ain’t it chief” and “weird flex but ok” commenters when they meet each other pic.twitter.com/VvVVKMWuaP
— ryan (@ryanyeetz) October 6, 2018
the Internet has ruined me in the sense that if a boy were to tell me he liked me I would probably just respond with “weird flex but ok” or “this ain’t it chief”
— jodie (@jodieegrace) October 1, 2018