To lamp someone is to smack them, usually with a proper punch, not a gentle tap. You’ll hear it as past tense too, like he lamped him, meaning he swung and connected. It implies sudden violence and a situation that’s already gone from banter to chaos. Bit grim, but very British in delivery.
"He got lamped outside the chippy after shouting dibs on the last battered sausage. Next thing you know, chips everywhere, his mate yelling leave it out, and a seagull nicking the evidence"