The word puny, mostly meaning "small" or "inferior" at this point, is a 1570s loanword from French. When it went into English, it meant "of a lower rank", so it's unsurprising that this comes from the word puisne, which meant "born after": younger people typically had lower ranks. Puisne is a portmanteau of puis ("after") and nez ("born", also the source of nee, pertaining to marriages), which is curious because it's rather small for such a combination. Puis is an alteration of the Latin word postea "after", from post, supposedly from the Proto-Indo-European word apo, "away". Nez or ne, as it sometimes appeared, is also a drastic shortening of Latin, this time from the word natus (the etymon of natal), meaning "birth". This is the past participle of nasci, "to be born", which derives from the word gnasci, which in turn traces to PIE gene "to give birth". The most interesting things we can glean from this are that puny has ageist connotations and it is quite the puny portmanteau.
Adam Aleksic, a rising sophomore studying linguistics and government at Harvard University, has been described as the internet's sixth most famous etymologist. He also has disturbing interests in politics, vexillology, geography, board games, conlanging, and law.