The phrase "not sure" expresses uncertainty or doubt about something. When a person says they are "not sure," it indicates that they do not have enough confidence or information to make a definitive statement or decision.
The word "sure" comes from the Old French "surs," meaning "secure" or "certain," which itself is derived from the Latin "securus," meaning "free from care" or "safe." The prefix "not" simply negates the meaning, indicating the absence of certainty.