A poon prune is when someones hand looks pruney like it's been soaked in water for too long after he or she has fingered someone repetitively. Hence, the word poon as in poon tang instead of vagina or pussy, since it rhymes with prune.
1. When a lady leaves her tampon in her vag too long...it turns into a poon prune.
2. The wrinkled vagina of an elderly person that has aged to perfection.
3. When your body displaces a bowel movement, and the excrement channels itself out of the vagina instead of the anus. The resulting waste is poon prune.