=Ox ford. The Thracian Bosphorus, or Bosporus, unites the Sea of Marmora with the Euxine (2 syl.) or Black Sea. According to Greek fable, Zeus (Jupiter) greatly loved Io, and changed her into a white cow or heifer from fear of Hera or Juno; to flee from whom she swam across the strait, which was thence called bos poros, the passage of the cow. Hera discovered the trick, and sent a gadfly to torment Io, who was made to wander, in a state of phrenzy, from land to land. The wanderings of Io were a favourite subject of story with the ancients. Ultimately, the persecuted Argive princess found rest on the banks of the Nile.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
More on Bosphorus from Fact Monster: