British slang for ‘on a drinking spree’, especially when drinking fizzy alcoholic drinks rather than spirits/liquors, and thus a synonym of various other similarly formed phrases like ‘On the piss/drink/lash/razz’. Green’s dictionary of slang also claims it can refer to taking drugs intravenously - I haven’t heard that but then I don’t hang around in those circles.
I was ‘on the pop’ last night. I drank 5 pints of beer, 2 pints of cider and an alcopop!