means The red, sore, sometimes peeling skin damage you get when you stay out in the sun too long without protection.

from A plain English compound of "sun" and "burn," both ancient Germanic words — "sun" related to Old English "sunne" and "burn" to "beornan," to be on fire. The pairing is straightforward and relatively modern, naming the obvious: the sun, doing to you what fire does. No mystery here, just two old words shaking hands over a shared talent for scorching.