Costly or expensive; requiring significant expenditure.
From Old French 'costeux,' derived from Latin 'costare.' An archaic English form, largely superseded by 'costly,' but preserving an older pronunciation and spelling convention.
This word sounds almost Shakespearean because it is—early English used '-ious' endings more freely, but over centuries we standardized to '-y' (costly), showing how languages gradually regularize their endings.
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