The act of blowing into a woman's anus while cupping your hand in front of her vagina as if to tune the note. Similar to the rusty trombone for a man.

When you eat dirt for the first time and leave your family to build a shack in the woods

Laying a girl on her side, and rubbing your lips back and forth across her vagina and anus while blowing simultaneously, treating her two love holes as you would the holes of a harmonica. To better create the rusty harmonica effect you may also cup her vagina lips and anus in your hand as you would hold a harmonica.

Giving a woman a rimjob, and a reach around. Often involves blowing in the butt cheeks.

When you actively blow air up the anus of your female partner, with the expectation of inhaling it back out of their ass while you rub her clit from behind for teh lulz.