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.
"Dude, I gave your mom the most righteous rusty harmonica last night!"
"Oh yea? What song did you play?"
"Blowin' in the Wind."
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.
I just gave her the ol' rusty harmonica.
Giving a woman a rimjob, and a reach around. Often involves blowing in the butt cheeks.
I gave your sister a rusty harmonica.
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.
Jenny wanted to try something kinky and new, so I surprised her with a Rusty Harmonica.