Apparently private dick or just dick are slang terms to refer to detectives, which is rather odd if you think about it. The usage was first recorded in a 1905 dictionary of slang, but was probably in use in casual speech for several decades before that. The origin is uncertain, but there are two main theories. It’s possible that it could come from a nineteenth-century criminal slang word meaning “to watch”. That would be unrelated to the other definitions of the term, eventually tracing to a Romany word spelled “dik” and meaning “to see", according to the Random House Historical Dictionary of American Slang. However, the Oxford English Dictionary thinks it’s just a playful shortening of the word “detective” (just remove a t and the -tive part) modelled after the nickname for “Richard”. The latter seems to be more widely accepted, but there’s no definitive proof supporting either.
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.