Definition: To ignore and/or pretend not to see something, usually something you know is wrong.
Origins: The origins of this idiom comes from the Battle of Copenhagen between the British Royal Navy and the Danish-Norwegian fleet near Copenhagen in 1801. Vice Admiral Horatio Nelson, leading the main British charge, had no sight in one eye from an incident earlier in his career. Admiral Sir Hyde Parker, the man in overall charge of the British forces, wanted Nelson to withdraw from fighting. Parker’s viewpoint of the battle was almost nonexistent because of the heavy smoke. From Parker’s perspective, the British were fighting a losing battle. So he had men relay his message via signal flags that demanded a response by Nelson who, in turn, had a better view of the battle. Believing they were in a good place, Nelson ignored Parker’s signals to retreat.
When the signals became more aggressive, Nelson picked up a telescope. Putting it over his blind eye, he reportedly said “I really do not see the signal.” By doing so, Nelson “turn[ed] a blind eye” and directly disobeyed the orders of his superior. The battle continued to rage. Both British and Danish fleets sustained heavy damage. Seeing this, Nelson sent a letter to Crown Prince Frederick (the Danish commander) for a truce which was accepted. How did Parker respond to his insubordinate Vice Admiral? With a truce in hand and the destruction of the majority of the Danish-Norwegian fleet around him, Parker realized that Nelson’s insistence on continual fighting was correct. The next day, Nelson was allowed to go into Copenhagen for the formal negotiations – something that is considered quite an honor.