Definition & Meaning of "[turn] a blind eye"

What is the origin of the idiom "turn a blind eye" and when to use it?

The origin of the idiom "turn a blind eye" can be traced back to the famous British Admiral Horatio Nelson. According to a widely accepted account, during the Battle of Copenhagen in 1801, Nelson's superior, Admiral Sir Hyde Parker, signaled for him to withdraw from the battle. However, Nelson, who had only one eye due to an earlier injury, held his telescope to his blind eye and famously said, "I really do not see the signal." He went on to achieve victory in the battle. This incident gave rise to the expression "to turn a blind eye," which means to deliberately ignore or overlook something. This expression is used to describe the act of ignoring or deliberately not noticing something, especially when it involves wrongdoing, rule-breaking, or misconduct. It implies a willful decision not to take action or intervene in a situation where one should.