What does Boxing Day have to do with boxing?
1834, "inflammation of the bursa;" also see -itis.
bursitis bur·si·tis (bər-sī'tĭs)n. Inflammation of a bursa, especially in the shoulder, elbow, or knee joint. Also called bursal synovitis.
Inflammation of a bursa. Common locations of bursitis include the joints of the shoulder, knee, and elbow.