Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
sub-sanno, āre, v. a. [sanna], to insult by derisive gestures, to deride, mock (late Lat.): ecce ipsi quasi subsannantes, Tert. adv. Jud. 11 med.; Hier. Ep. 40, 2; Vulg. Psa. 34, 16; id. Isa. 37, 22.