Is it ensure, insure, or assure?
scornful, mocking; disdainfully humorous
Greek sardonios 'derisive'
"apparently but not really proceeding from gaiety," 1630s, from French sardonique (16c.), from Latin sardonius (but as if from Latin *sardonicus) in Sardonius risus, loan-translation of Greek sardonios (gelos) "of bitter or scornful (laughter)," altered from Homeric sardanios (of uncertain origin) by influence of Sardonios "Sardinian," because the Greeks believed that eating a certain plant they called sardonion (literally "plant from Sardinia," see Sardinia) caused facial convulsions resembling those of sardonic laughter, usually followed by death. For nuances of usage, see humor. Earlier in same sense sardonian (1580s), from Latin sardonius. Related: Sardonically.