Sap is an American slang term for an idiot or sucker. According to Daniel Cassidy, the dork of New York, in his crap book of fake etymologies, How The Irish Invented Slang, this derives from the Irish sop, which according to him means ‘a wisp of straw, a useless lout, a cowardly weak fellow, a silly person.’ This sounds like a good match, doesn’t it? Unfortunately, Cassidy seems to have made this definition up, as Dinneen’s dictionary gives no human meanings at all for sop, while Ó Dónaill’s dictionary states that sop de dhuine (a sop of a person) can mean ‘a wispy or unkempt person.’ Most of the lengthy definition quoted by Cassidy seems to be pure invention.
Back in the real world, sap seems to derive from earlier English and Scottish terms like sapskull and saphead, both of them meaning chump or idiot. You can find out more here at the excellent Online Etymological Dictionary, a resource created by real scholars to inform people about genuine word origins: