'unpierced' is formed from the prefix 'un-' (meaning 'not') plus the past participle 'pierced', where 'pierce' originates from Old French 'percer' (to pierce) ultimately from Latin elements related to striking or making a hole.

'pierce' changed from Old French 'percer' into Middle English forms (e.g. 'percen') and eventually became modern English 'pierce'; the negative prefix 'un-' comes from Old English 'un-'. These combined to give the adjective 'unpierced' meaning 'not pierced'.

Initially related to the action 'to make a hole through' (pierce); over time the derived form with 'un-' came to mean 'not having been pierced' or 'not perforated'.