Here's why:
* Colloquial: This refers to informal language used in everyday conversation.
* Root word: This is the base form of a word, often with a specific meaning.
* Mean: In colloquial speech, "mean" can be used to describe someone who is unkind or cruel.
* Un-mean: This prefix changes the meaning to "not mean" or "kind."
Therefore, the prefix "un-" modifies the root word "mean" to create the opposite meaning in informal language.