a sucker punch, especially one that knocks somebody out cold.
It's also a Merle Haggard if your friend pretends to check on the unconscious victim and instead starts delivering a fury of vicious blows to the already injured person.
Inspired by the true story of a bar room brawl involving Merle Haggard, his first wife, a bandmate, and a really big, bad, mean fellow.
I didn't have a chance against that huge goon, so I played it cool until he turned his back on me, then I gave that sorry motherfucker a Merle Haggard like he's never seen. Then my friend gave him a Merle Haggard after he was already knocked out cold.