What part of speech is about in the sentence The man wandered about?

I'm about 90% certain it is an adverb. About can serve as an adverb, preposition or adjective. I'm 99.9999999999999% certain it isn't an adjective as used here, but preposition or adverb depends on whether there is anything more to the sentence: He wandered about (aimlessly) => about is an adverb He wandered about the park => about is a preposition