Io

Webster's Dictionary

(n.) An exclamation of joy or triumph; - often interjectional.

The Nuttall Encyclopedia

In the Greek mythology a daughter of Inachos ( q. v .), beloved by Zeus, whom Hera out of jealousy changed into a heifer and set the hundred-eyed Argus to watch, but when Zeus had by Hermes slain the watcher, Hera sent a gadfly to goad over the world, over which she ranged distractedly till she reached Egypt, where Osiris married her, and was in connection with him worshipped as Isis.