Here's why:
* Adverbs describe verbs, adjectives, or other adverbs. They tell us how, when, where, or to what extent something is done.
* Feebly describes the manner in which something is done, indicating a lack of strength or energy.
For example:
* "The old man walked feebly down the street." (Describes how he walked)
* "She spoke feebly in a barely audible voice." (Describes how she spoke)