I'm going to go out on a limb here and say that this is only an example of slang: words that are not considered part of the standard vocabulary of a language and that are used very informally in speech especially by a particular group of people. - MW
It is certainly alliterative: the repetition of the B sound, but other than that, I can't really think that there is another literary device at work here.
Synecdoche is use of a part to symbolize a whole, e.g. "All hands on deck" means every crew member report to the deck. Conversely, using the whole to refer to a part, if one hears that, say, Tennessee is in the lead in a basketball game, we know that it's not all of Tennessee, but merely a team.
Metonymy is when the word use to describe another thing is closely linked to that particular thing, but is not a part of it. For example, "give me a hand" means "give me some help". Hands are usually involved, but the metonymy is using hand for help.
I'm not sure it's a metaphor (though it might be). A metaphor for blue eyes might be "azure pools to drown in" (sorry, I'm not a writer).
Oh, and as to part of speech, it functins as an adjective + noun in your sentence.