Here's why:
* It modifies verbs, adjectives, or other adverbs: It tells us how much or to what extent something is done or is true.
* Example: "He entirely forgot the meeting." (modifies the verb "forgot")
* Example: "The room was entirely empty." (modifies the adjective "empty")
* Example: "She is entirely too busy to help." (modifies the adverb "too")
Let me know if you have any other words you'd like to categorize!