What does Boxing Day have to do with boxing?
mid-16c., originally "spread out" (hair), of unknown origin, perhaps from Dutch vlederen. Related: Flared; flaring. The notion of "spreading out in display" is behind the notion of "spreading gradually outward" (1640s). Flare-up "a sudden burst" is from 1837.
"bright, unsteady light," 1814, from flare (v.), which led to the sense of "signal fire" (1883). Flares "flared trousers" is from 1964.
An area of redness on the skin surrounding the primary site of infection or irritation.