In the days before modern harbors, a ship had to wait for the flood tide before it could make it to port. The term for this situation in Latin was ob portu, that is, a ship standing over off a port, waiting for the moment when it could ride the turn of the tide to harbor.
The English word opportunity is derived from this original meaning. The captain and the crew were ready and waiting for that one moment for they knew that if they missed it, they would have to wait for another tide to come in. Shakespeare turned this background of the exact meaning of opportunity into one of his most famous passages. Its from Julius Caesar, Act 4, Scene 3:
There is a tide in the affairs of men,
Which, taken at the flood, leads on to fortune;
Omitted, all the voyage of their life
Is bound in shallows and in miseries.
On such a full sea are we now afloat;
And we must take the current when it serves,
Or lose our ventures.