What does Boxing Day have to do with boxing?
late 14c., present participle adjective from roar (v.). Used of periods of years characterized by noisy revelry, especially roaring twenties (1930); but also, in Britain, roaring fifties (1892). Roaring forties in reference to exceptional rough seas between latitudes 40 and 50 south, is attested from 1841.
Old English rarian "roar, wail, lament, bellow, cry," probably of imitative origin (cf. Middle Dutch reeren, German röhren "to roar;" Sanskrit ragati "barks;" Lithuanian reju "to scold;" Old Church Slavonic revo "I roar;" Latin raucus "hoarse"). Related: Roared; roaring.
late 14c., from roar (v.) and Old English gerar.