Its used to symbolize fear; generally meaning that they are on the verge of explosion.
Its a Urdu language word and has originated from the streets. Many kids/teens in Pakistan and India use it.
Its considered as slang language
'i tried to convice him to that he should complainto theprinciple but uski phat gai'
'jab bhi woh apne abu ko dekhe uski phat jati hai'
pick me boy: STAWPPP BEHAVE YOUR SELF! if anything u have the phat ass but u prob think im weird for saying that
swag girl: yeah that ws pretty weird. *turns around and blushes secretly
*