turn a blind eye

Probably from the idea of a person turning to look at something but not seeing it, as if their eyes are blind.

The term is frequently claimed to originate from an incident during the First Battle of Copenhagen on 2 April 1801, when Vice Admiral Horatio Nelson (1758–1805) was ordered by Admiral Hyde Parker (1739–1807) through signal flags to discontinue naval action against a force of the Dano-Norwegian Navy. Nelson, who had been blinded in one eye early in his career, said to his flag captain Thomas Foley (1757–1833), “You know, Foley, I have only one eye—I have a right to be blind sometimes.” He then put his telescope to his blind eye and, remarking “I really do not see the signal,” continued the assault which ended in a British victory. However, this is not the source of the term as the Oxford English Dictionary records uses dating to the 17th and 18th centuries.