Acerb, a.
Etymology: [L. acerbus, fr. acer sharp: cf. F. acerbe. See Acrid.]
Sour, bitter, and harsh to the taste, as unripe fruit; sharp and harsh.
Shades of meaning
Fernald has no entry of its own for acerb; he lists it under bitter.
He lists it without comment; the entries above explain its neighbors.