Lewis & Short

excē̆tra, ae, f. [perh. corrupted from ἔχιδνα], a snake, serpent (very rare).

  1. I. Prop., Plaut. Pers. 1, 1, 3 sq.; * Cic. poet. Tusc. 2, 9, 22.
  2. II. Transf., a reproachful epithet for a bad, intriguing woman, Plaut. Cas. 3, 5, 19; id. Ps. 1, 2, 82; Liv. 39, 11.