That girl know how to blow
Something like she played the flute

from Mac Miller – Wear My Hat Lyrics on Rap Genius

As you know, a flute works by pressing down on the keys and blowing into a small hole. Without being able to blow into this hole, no music can be played.

However, Mac is using a play on words with the flute and by blowing, he is saying she knows how to give amazing blowjobs.

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