A short form of the phease "hand job". A sexual practice, usually first practived in the pre- or early teen years, where a girl masterbates a guy with her hand. Handies often lead to harder forms of sex, such as blow jobs, which initially is just a handie where she puts her mouth over the top of the penis until he ejaculates into her mouth instead of into the air.
Bob's little sister Paula was teasing me so much that I finally told her to either stop it or she'd have to give me a handie to take care of the arousal she caused. Being curious about what a handie was, she let me teach her how to do it. She even took her top off so I could cum on her developing breasts. A month later, a handie led to me cumming in her mouth.