Scottish National Dictionary (1700–)
†LUM, v.2, n.2
I. v. Of rain: to fall in a downpour.
Gall. 1824 MacTaggart Gallov. Encycl. 325:
The rain is just coming lumming down.
II. n. In phr. a lum of a day, a very wet day (Ib.).[Appar. the same word as Midl.Eng. dial. lomb, a downpour, waterfall, phs. also lum(b), a water-hole in a mire, a deep pool, and maybe ultimately of the same orig. as Lum, n.1, v.1]
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"Lum v.2, n.2". Dictionary of the Scots Language. 2004. Scottish Language Dictionaries Ltd. Accessed 26 May 2018 <http://www.dsl.ac.uk/entry/snd/lum_v2_n2>
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