What does Boxing Day have to do with boxing?
c.1300 as a verb, "to look sullen or displeased," perhaps from Scandinavian (cf. Norwegian dialectal glome "to stare somberly"). Not considered to be related to Old English glom "twilight," but perhaps to Middle Low German glum "turbid," Dutch gluren "to leer." The noun is 1590s in Scottish, "sullen look," from the verb. Sense of "darkness, obscurity" is first recorded 1629 in Milton's poetry; that of "melancholy" is 1744 (gloomy in this sense is attested from 1580s).