'puny' originates from Middle English, ultimately from Old French 'puisné' (also spelled 'puisne'), where the components came from Latin roots meaning 'born later' (post + natus) implying 'younger' or 'inferior'.

'puny' changed from Middle English forms such as 'puisne'/'puisny' (borrowed from Old French 'puisné') and, through phonetic alteration and folk usage in Early Modern English, became the modern form 'puny' with the sense of 'small' or 'weak'.

Initially it meant 'younger' or 'born later' (hence 'inferior' in rank); over time the sense shifted to 'weak, small, or insignificant', which is the common modern meaning.