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representing Old English glómung str. fem., from (on the analogy of ǽfning evening) glóm twilight, probably from the Teut. root *glô-; the etymological sense would thus seem to be the ‘glow’ of sunset or sunrise, whence the passage to the recorded sense is not difficult.
From: Ancient and Modern Scottish Songs, Heroic Ballads, Etc.
By David Herd, Volume the Second, 1776
Appendix, P. 46