To pop somewhere quickly and come straight back, usually for one specific mission like grabbing milk, posting a letter, or using the loo. It implies it’ll be brief, which makes it perfect for the classic lie where you vanish for ages. Proper everyday UK phrase, said like it’s nothing, even when it’s absolutely not nothing.
"I’m just nipping to the shops for a pint of milk, yeah? Sound. Forty minutes later he’s back with a meal deal, scratchcards, and a fresh tan from the offie queue"