'slowly-widened' originates from English, combining the adverb 'slowly' (from Old English 'slaw'/'slǣw' with the adverbial/derivational element that became '-ly') and the verb 'widen' (from Old English 'wīdian', based on 'wīd' meaning 'wide').

'slowly' evolved from Old English forms such as 'slaw' + suffixes into Middle English 'slowliche'/'slowly', and modern English 'slowly'; 'widen' changed from Old English 'wīdian' to Middle English 'widen' and later formed past tense/past participle 'widened', which together yield the compound descriptive form 'slowly-widened'.

Initially, 'slow' and 'wide' referred separately to slowness and breadth; combined as a verb phrase they historically meant 'to become wide', and in modern combined usage the compound describes the process as occurring gradually—'became wider slowly'.