To furnish with a ceiling; to cover the inside top of a room.
From Middle English 'celen,' derived from Old French 'ciel' meaning 'sky' or 'heaven,' from Latin 'caelum.' Originally meant the sky, then evolved to mean the roof/covering of a room because it's overhead like the sky.
The word 'ceiling' comes from the same root as 'celestial'—both trace back to Latin 'caelum' (sky)—so in a way, when you look up at your ceiling, you're looking at your indoor 'sky'!
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