To provide or supply something needed; to equip with furniture, equipment, or necessary items.
From Old French 'fournir' meaning 'to complete' or 'to provide', possibly from a Germanic source meaning 'to further' or 'advance'. The word originally meant to complete or make ready, which developed into the specific sense of providing furniture and the general sense of supplying what is needed.
We think of 'furnish' as putting furniture in a room, but it originally meant 'to complete' or 'make ready'! This explains why we can furnish information, furnish proof, or furnish supplies - you're completing what someone needs, whether that's a room, an argument, or a project.
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