"Despisable" means "despicable", but is archaic (archaic=no longer in everyday use but sometimes used to impart an old-fashioned flavour). It does however occasionally happen that people use archaic words, for instance was "depisable" used in 2014 in this article http://www.bestofneworleans.com/gambit/interview-john-lydon/Content?oid=2449933

Forget about 'despisable'. It's never used.