In a fit, suitable, or appropriate manner; properly and well.
From 'fit' (suitable, proper) plus '-ly' (in the manner of), creating an adverb dating back to Middle English. The root 'fit' originally meant 'fitting together' or 'suitable.'
Though fitly is now quite archaic, it appears in older literature to mean 'appropriately'—Shakespeare and Bible translators used it to show that something happened exactly as it should have.
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