Here's why:
* It modifies a verb: It tells us how something was done or how something happened.
* Example: "He originally planned to go to college, but changed his mind." ("originally" modifies the verb "planned")
* It can also modify an adjective or another adverb:
* Example: "The house is originally from the 19th century." ("originally" modifies the adjective "original")
Let me know if you have any other words you'd like to know the part of speech for!