The phrase "to turn a blind eye" means to deliberately ignore something, usually something wrong or undesirable. The origin of this phrase is attributed to a specific historical event involving the British naval hero Admiral Horatio Nelson. 

Here's a detailed look at its origin: The expression is believed to have originated during the Battle of Copenhagen in 1801, part of the Napoleonic Wars. Admiral Horatio Nelson was second in command of the British fleet under Admiral Sir Hyde Parker. 

During the battle, Parker, who was more cautious, signaled Nelson's forces to withdraw by raising the signal flags for retreat. Nelson, who was known for his aggressive tactics, wanted to continue the attack despite the order to retreat. He famously put his telescope to his blind eye (Nelson had lost sight in one eye earlier in his career) and claimed not to see the signal. 

By doing this, he could plausibly deny having seen the order and continued his assault, which eventually led to a significant British victory.