What does Boxing Day have to do with boxing?
1806, from Modern Latin chorea Sancti Viti "St. Vitus dance" (originally a mass hysteria in 15c. Europe characterized by uncontrolled dancing); from Latin chorea "a dance," from Greek khoreia "dance" (see chorus). Extension to the nerve disorder is from 1620s.
chorea cho·re·a (kô-rē'ə, kə-)
Irregular, spasmodic, involuntary movements of the limbs or facial muscles.