'uncommonly-mentioned' originates from Modern English, specifically the words 'uncommonly' and 'mentioned', where the prefix 'un-' meant 'not', 'commonly' derived from 'common' meant 'generally', and 'mention' derived from Latin 'mentio' meaning 'a calling to mind'.

'uncommonly-mentioned' developed as a compound adjective in Modern English from 'uncommonly' + 'mentioned'. 'Uncommonly' was formed from the Old English/Proto-Germanic negative prefix 'un-' plus Middle English 'commonly' (from Old French 'comun' and Latin 'communis'). 'Mentioned' is the past participle of 'mention', which came into Middle English from Old French 'mencion' (or 'mencioner') and ultimately from Latin 'mentio, mentionis'.

Initially its parts conveyed 'not in a common or usual manner' and 'called to mind'; over time the compound came to be used to mean simply 'rarely mentioned' or 'not often referred to' in modern usage.