A phrase used to represent someone that is always saying odd things, or things out of context with the original topic of conversation. This is also a term used for really smart people who always share their knowledge about a specific topic, whether they were asked to or not.
When you ask someone to hit the button for your floor on the elevator and they say. I was 14 once. I remember it well, I got a bike for my birthday that year. You say to them, you are a strange cookie.