A more thorough explanation:

EISNE

Eisne (ayn) is a term from Law French that means "eldest" or "firstborn." It can also refer to an illegitimate son whose parents later marry and have a second son for lawful issue.

One example of eisne is when a family has two sons, but the first son was born out of wedlock. When the parents later marry and have a second son, the second son is considered the lawful heir, while the first son is the bastard eisne.

Another example is when a person is referred to as the eisne of a family, meaning they are the oldest or firstborn child.

These examples illustrate how the term eisne is used to describe the eldest or firstborn child, as well as the specific situation of an illegitimate son in a family with a later lawful heir.