Dreadful, a.
1. Full of dread or terror; fearful. [Obs.] "With dreadful heart." Chaucer.
2. Inspiring dread; impressing great fear; fearful; terrible; as, a dreadful storm. " Dreadful gloom." Milton. For all things are less dreadful than they seem. Wordsworth.
3. Inspiring awe or reverence; awful. [Obs.] "God's dreadful law." Shak.
Syn. -- Fearful; frightful; terrific; terrible; horrible; horrid; formidable; tremendous; awful; venerable. See Frightful.
Shades of meaning
Fernald has no entry of its own for dreadful; he lists it under awful.
He lists it without comment; the entries above explain its neighbors.