'prosaize' originates from Modern English, specifically formed from the adjective 'prosaic' plus the verb-forming suffix '-ize', where 'prosaic' ultimately comes from Latin 'prosaicus' meaning 'in prose' and the suffix '-ize' means 'to make or to become'.

'prosaize' developed in modern English by combining 'prosaic' (from French 'prosaïque' and Latin 'prosaicus') with the productive English suffix '-ize'; earlier senses related to making something prose-like or commonplace trace back to Late Latin and Old French terms for prose and plain speech.

Initially it carried the narrow sense 'to render into prose' (that is, to convert verse to prose); over time it acquired the broader, figurative sense 'to make dull or commonplace', which is common in modern usage.