Forthwith ( or ; see With), adv.
1. Immediately; without delay; directly. Immediately there fell from his eyes as it had been scales; and he received sight forthwith. Acts ix. 18.
2. (Law)
As soon as the thing required may be done by reasonable exertion confined to that object. Bouvier.
Shades of meaning
Fernald has no entry of its own for forthwith; he lists it under immediately.
He lists it without comment; the entries above explain its neighbors.