Means a quick look or a nosy little peek, as in have a butcher’s. Comes from Cockney rhyming slang butcher’s hook = look, then clipped to butcher’s because nobody’s got time for the full thing. You’ll hear it when someone wants you to check something out, clock some drama, or admire a bargain. Best served with a cheeky tone and an oi.
"Oi, have a butcher’s at this dodgy kebab I found on the Tube platform. If it moves, I’m off, bruv."