A term used by technically clueless people to refer to portable storage devices because it is the letter that Windows assigns to it when they plug it in. They could similarly use other letters depending on their common system configuration such as "d", "f", or "g" -drive.

Briefly, "the realization that each random passerby is living a life as vivid and complex as your own." Originally from the Dictionary of Obscure Sorrows, which has a lot more beautiful neologism definitions like this you might enjoy.