to make a move on a someone who is very sleepy or was asleep, before you woke him/her up to make a sexual advance. Jape is generally successful because when sleepy, the japee is less inhibited and more likely to indulge in instinctive sexual desires.
esp. when the japer is at first with the japee in a non predatory (platonic) way. The more adorable and harmless the japer is, the greater the degree of jape.
we were watching a movie and she started falling asleep but i really wanted to hook up. so i japed her. she really liked me though so it's ok.