While having sex, when you feel the urge, you pull out and begin to ejaculate upon your partner's stomach, or tummy, thus giving them a "cummy".
Harry:"Last night was something else."
John: "Oh yeah, what happened?"
Harry: "Cindy came over and we had some sex. I didn't have a condom and she wasn't on the pill so i pulled out and gave her a cummy. Shit was so cash."