1)The susbtitute for a word which is indescribable, or ineffiable. If someone has done something, but no other word seems to fill the gap, pronnop shows that your feelings are big, and that no other word can define your emotion.
2)it can mean anything you want it to mean, but must be said with complete assurance!
don't be such a .... pronnop!
That was so pronnop of you to do that!