'unfelled' originates from Old English elements: the negative prefix 'un-' (Old English 'un-') where 'un-' meant 'not', combined with the verb 'fell' from Old English 'fellan', where 'fellan' meant 'to cause to fall or to cut down'.

'unfelled' developed by combining the longstanding negative prefix 'un-' with the past-participle form of Middle English 'fellen'/'fell' (from Old English 'fellan'); this yielded Middle English formations like 'unfelled' which evolved into the modern English adjective and past participle 'unfelled'.

Initially it meant 'not caused to fall or to be cut down'; over time the phrase retained this core sense and is used in modern English to mean 'not felled' (especially of trees) or 'not knocked down'.