Unavoidable, a. — Webster enters it under “unavoidable”
1. Not avoidable; incapable of being shunned or prevented; inevitable; necessary; as, unavoidable troubles.
2. (Law)
Not voidable; incapable of being made null or void. Blackstone. Unavoidable hemorrhage (Med.), hemorrhage produced by the afterbirth, or placenta, being situated over the mouth of the womb so as to require detachment before the child can be born. -- Un`a*void"a*ble*ness, n. -- Un`a*void"a*bly, adv.
Shades of meaning
Fernald has no entry of its own for unavoidableness; he lists it under necessity.
He lists it without comment; the entries above explain its neighbors.