'rosily' originates from English, formed by adding the adverbial suffix '-ly' to the adjective 'rosy', where the suffix '-ly' meant 'in the manner of' or 'like'.

'rosily' was created in Modern English by attaching '-ly' to 'rosy'. The adjective 'rosy' itself developed from Middle English 'rosy' (from Old French 'rose' or 'rosé'), which ultimately comes from Latin 'rosa' meaning 'rose (the flower)'.

Initially related to the color or appearance of a rose (pinkish or rosy complexion), the word extended figuratively to mean a hopeful or optimistic outlook; this figurative sense became common in modern usage.