What is a common meaning of the prefix 'un-' in English?

The prefix 'un-' commonly means 'not' in English. It is used to negate or reverse the meaning of the base word it is attached to. For example, if you take the word 'happy' and add the prefix 'un-' to it, the new word 'unhappy' means 'not happy.' This prefix is used in various other words as well, such as 'unlock,' meaning 'not locked,' or 'unknown,' meaning 'not known.' The use of 'un-' effectively changes the meaning to the opposite, which is why this choice accurately reflects the common meaning of the prefix.