Same entry in other lexica (of which one, Etymologicum Gudianum 496.50, has a slightly longer gloss: "laughing with bitterness and anger"); references at Photius sigma81 Theodoridis.
The headword as transmitted is a present participle (in the masculine nominative singular), seemingly quoted from somewhere. A verb sarda/zw is otherwise unattested, but no matter -- this is an error for what should be sarka/zwn: see sigma 133 (which begins with the Etymologicum Gudianum gloss) and generally LSJ s.v.