(adj.) junior, of inferior rank
There’s an odd corner of our language where established French phrases are ran together as one unit, and the result then added to our English vocabulary as a single word.
Embonpoint, meaning plumpness, for instance, comes from en bon point (literally ‘in good shape’). Do something point-blank, and you’re really doing it de pointe en blanc (a phrase likely alluding to a weapon being fired straight forward, into open space). The French legal tag culpable, prest (‘guilty, ready [to prove our case]’) is the origin of our word culprit. And an affair was originally an à faire (‘something to be done’—though admittedly, that had already been ran rather as affaire in French before we picked it up).
To this list, we can also add both the rather formal adjective puisne, meaning low-ranking, and the much more familiar adjective, puny.
Puis né essentially means ‘born afterwards’ in French, and since Norman times has been used to designate younger siblings in family records, and, more figuratively, junior rather than senior appointees in legal documentation and other official papers. Puis né was used so often in instances like these that it gradually ran together as a single unit, puisne, which was then picked up and adopted into English in the 1500s.
But some English speakers weren’t quite so au fait with their French, so while puisne has survived as a word in its own right in English (albeit still in fairly formal and official contexts), we also took it a step further. Puis né became puisne, and in turn—in the sense of something or someone weak, undeveloped, low-ranking or otherwise at a disadvantage—it became puny.