The word "too" is an adverb that means "also" or "as well" when used in a positive sense, or it indicates excessiveness when used in a negative context. For example, when we say "I want to go too," it means "I want to go as well." In contrast, saying "It's too hot" implies that the heat is excessive.
The word "too" originates from Old English "to," which means "in addition" and is related to the Old Norse "til," which means "to" or "towards." The use of "too" to indicate excessiveness developed over time.