To turn a blind eye is to knowingly pretend not to see or notice something, usually something problematic or prohibited. It implies a choice — not genuine unawareness, but a deliberate decision to act as though something is not happening. A manager who turns a blind eye to time-theft enables it. A customs officer who turns a blind eye to smuggling is complicit in it.
The phrase is almost universally traced to Vice Admiral Horatio Nelson at the Battle of Copenhagen in 1801. During a complex naval engagement, Nelson's superior Admiral Hyde Parker sent a signal flag ordering the fleet to disengage. Nelson, who had been blinded in one eye during a battle years earlier, reportedly put his telescope to his blind eye and said: 'I have a right to be blind sometimes. I really do not see the signal.' He pressed the attack and won a decisive victory.
The account of Nelson's blind eye has been told in different versions, and some historians question the exact words attributed to him. However, contemporaneous accounts and letters from officers present at the battle confirm that he deliberately disregarded the signal. Whether or not the telescope story is perfectly accurate, the event was widely reported and directly generated the phrase.
Nelson lost the sight in his right eye in 1794 during the Siege of Calvi in Corsica, when debris from a shot that struck the ground near him damaged his eye. He never wore an eye patch — that embellishment comes from later theatrical representations.
They are close synonyms. 'Looking the other way' is slightly more neutral; 'turning a blind eye' implies a stronger element of knowing complicity — you are aware something is wrong and actively choosing not to address it.