There is an old thread on this topic. It looks they mostly means juicy feeling that comes from combination of youthful fruit and acidity. It also seems used more by Burg drinkers so I suspect one of the Burg critics frequently use this description which propagated to a certain group of wine drinkers, but I’m not very sure about that.
https://www.wineberserkers.com/t/what-does-the-term-sap-or-sappy-mean-to-you/33425