While onomatopoeias can function as different parts of speech depending on how they're used in a sentence, "meow" is most commonly used as a verb:
* The cat meowed loudly.
However, "meow" can also be used as a noun:
* I heard a soft meow from the kitten.
It's worth noting that "meow" doesn't have a specific grammatical function like a verb or noun. It primarily serves to represent a sound.