Third person singular present tense of begloom; covers or surrounds with gloom or darkness.
From be- + gloom + -s (third person singular marker). See beglooming for full etymology of the root.
This is what's called a 'productive' verb formation in English—the be- prefix could be attached to almost any adjective in historical English, creating dozens of now-archaic verbs that expressed the idea of 'becoming' or 'making' that quality.
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