A more thorough explanation:

Bastard eisne: An illegitimate son whose parents later marry and have a second son (mulier puisne) for lawful issue. The term "eisne" means "eldest" or "firstborn" in Law French.

For example, if a man has a son out of wedlock and later marries the child's mother, and they have another son who is born after the marriage, the first son would be considered a "bastard eisne." The second son, born after the marriage, would be the "mulier puisne" or legitimate heir.

Another example would be if a woman has a child out of wedlock and later marries the child's father, and they have another child who is born after the marriage. The first child would be the "bastard eisne," and the second child would be the legitimate heir.

These examples illustrate the concept of "bastard eisne" where the firstborn child is illegitimate but may still have some rights to inheritance or succession if the parents later marry and have a legitimate child.