When girls lip something (mouthread) and whisper something at the same time in public, trying to talk shit or tell a secret. It usually is never interpreted correctly by the person they are talking to, because doing both at the same time makes no sense, and everyone around you knows what you are trying to do. Not sly.
Those girls in the corner are lispering.
Julie is trying to lisper to Noelle and Noelle is trying to pretend she knows what she is trying to say.
Leigh lispered very loud at the party and people caught on!