What does Boxing Day have to do with boxing?
1520s, from sun (n.) + burn (v.). Sunburnt (c.1400) is older than sunburned (c.1500, sunne y-brent). As a noun from 1650s.
sunburn sun·burn (sŭn'bûrn')n. Inflammation and erythema of the skin, often with blistering, caused by overexposure to the ultraviolet rays of direct sunlight.