A more thorough explanation:

Definition: Mulier puisne is a term from Law Latin that refers to the younger lawful son, usually distinguished from the elder bastard son (bastard eigné).

For example, in the common law, if the eldest son was born out of wedlock (bastard eigné) and the next son was born to the same parents after the marriage (mulier puisné), and upon the ancestor's death the bastard eigné entered as heir and remained in undisturbed possession until his own death, the bastard eigné was treated as if he had been legitimate with respect to the inheritance of that land.

This means that even though the eldest son was born out of wedlock, he would still be considered legitimate for the purpose of inheriting the land if he was the only son at the time of his father's death. This exception was made because a person who was legitimate by the Canon law could not be declared a bastard after their death, when they no longer had the opportunity to contest the issue.