Lewis & Short

1. ĭō, interj. [ἰώ], expressing joy,

  1. I. ho! huzza! hurra! io hymen hymenaee, io hymen, Plaut. Cas. 4, 3, 3; id. Ps. 2, 4, 11: miles, io, magna voce, triumphe, canet, Tib. 2, 5, 121 (118): io triumphe! Hor. C. 4, 2, 49: io, io, liber ad te venio, Plin. Ep. 3, 9.
  2. II. Expressing pain, oh! ah! io! enicas me miserum, Plaut. Truc. 1, 2, 21: uror, io, remove saeva puella faces! Tib. 2, 4, 6.
  3. III. Used in a sudden or vehement call, holla! look! quick! io! matres, audite, Verg. A. 7, 400: io! comites, his retia tendite silvis, Ov. M. 4, 513; id. A. A. 3, 742.