To make sacred or holy; to hallow or consecrate something with respect and ceremony.
Be- (causative) + hallow (from Old English halgian, from halig 'holy'). The prefix be- turns the adjective-root into a causative verb, making something become hallowed or sacred.
Behallow is barely used today, but Halloween is literally 'All Hallows' Eve'—same 'hallow' root—showing how the sacred/holy sense of 'hallow' survives in modern English even though we rarely use 'behallow' to describe making things sacred.
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