Here's why:
* It modifies a verb, adjective, or another adverb, describing how something is done.
* It comes from the adjective "sage," which means wise.
* When used as an adverb, "sagely" means "in a wise or insightful manner."
Example:
"The old man smiled sagely, his eyes twinkling with wisdom."
(Here, "sagely" modifies the verb "smiled," describing how he smiled.)