What part of speech is 'quickly'...

[evelynevelyn]

… in the following sentence:

The sign board read: ‘Quickly!’.

Also, the question mark would be at the end, where you have a full stop. But it makes sense without.

I think the word quickly is standing in for an imperative sentence. It's been shortened to an instruction to do something (i.e., the core imperative verb) so the meaning is probably "Go quickly!" rather than quickly. So imho it's functioning as a verb in this sentence.

I think part the problem here is that "Quickly!" by itself isn't really a sentence - it doesn't express a 'complete thought'. On the sign board there's probably some context so it makes some sense. In the context of the OP's sentence The sign board read: ‘Quickly!’. all it is is some text from a sign that's been quoted.

Thank you all. Very interesting.

Having read what you all put this morning, I'm with imperative adverb.

Come quickly! Go quickly! Eat your breakfast quickly!