The Brief: Bet is a slang term that acknowledges agreement. It is abbreviated from "you can bet on it".
DEEP DIVE
In response to a statement or questions, “Bet” means “Okay” or “Yaaasss”. The term comes from similar phrases like “you bet” or “you can bet on it”, all of which imply agreement.
friend:”you won’t “
me: BET https://t.co/IzTRcTOEwk— ℭ𝔬𝔷𝔶 𝔑𝔞𝔫𝔷 ☁️ (@c0zytapes) March 27, 2019