Green’s Dictionary of Slang

as if phr.

(UK black) used to emphasise one's feeling or action (reverse of the SE phr. which is used for denial).

hubpages.com ‘Roadman Slang’ 10 Jan. 🌐 As if... [...] a genuine English phrase that has been warped slightly in its usage. You would say ‘as if I just ate 3 packs of Oreos!’. That phrase alone would be a comical way of saying ‘I ate 3 packs of oreos guys!’. It can also be used in an exasperated way (and this is more frequently seen). A girl might angrily say ‘as if my boyfriend just told me he's about to go on holiday for 8 weeks’, meaning ‘I'm pissed because he's going away without me’.