Is the word weakly an adverb?

Yes, "weakly" is an adverb. It modifies verbs, adjectives, and other adverbs, describing how something is done or to what extent.

Here are some examples:

* "He shook his head weakly." (modifies the verb "shook")

* "The light was shining weakly through the fog." (modifies the adjective "shining")

* "She smiled weakly at him." (modifies the verb "smiled")

However, it's important to note that "weakly" can also be an adjective, meaning "lacking strength" or "not strong". For example: "The patient was feeling weakly."

So, the part of speech of "weakly" depends on its context within a sentence.