Emily Dickinson Lexicon

SHEATH, n. [Sax. sceath, scæthe; G. scheide; D. scheede; from separating, G. scheiden, D. scheien; Sax. sceadan. See Shade.]

1. A case for the reception of a sword or other long and slender instrument; a scabbard. A sheath is that which separates, and hence a defense.
2. In botany, a membrane investing a stem or branch, as in grasses. – Martyn.
3. Any thin covering for defense; the wing-case of an insect.