As late as the year 1808, some people were spelling the word eye as ee. Other alterations throughout history include eage, eygh, yee, ene, eie, eyȝnen and eiȝe with yoghs, nye, ney, neye, and nie (the last four because of rebracketing "an eye" into "a neye" or something equivalent in the seventeenth century, but that died out quickly), and more. As such a simple, common word, it sure changed a lot. Through Old English ege, it all traces to Proto-Germanic augo and ultimately Proto-Indo-European hek, with the same definition. The earliest use of the phrase private eye in print was in a 1938 edition of Dime Detective, a pulp fiction magazine. It was a pun based off the abbreviation of private I for private investigator and the idea that PIs spy on people, hence the eye part.
Adam Aleksic, a rising sophomore studying linguistics and government at Harvard University, has been described as the internet's sixth most famous etymologist. He also has disturbing interests in politics, vexillology, geography, board games, conlanging, and law.