A "sap" is slang for a stupid or pathetic person. Sometimes, depending on the context, "sap" could mean a sentimental or emotional person. You may hear someone say, "that's so sappy!" when referring to a romantic scene in a movie. Hope this helped!

Say you see a romantic scene in a movie and you say "AWWWWW. That is so sweet / cute. I want a love like that!" Someone might call you a sap because they essentially think you are being lame for being so emotional over it. :)