Jamieson, Fausset and Brown on 1 Samuel 21:13

13. feigned himself mad--It is supposed to have been an attack of epilepsy, real or perhaps only pretended. This disease is relieved by foaming at the mouth.

let his spittle fall down upon his beard--No wonder that Achish supposed him insane, as such an indignity, whether done by another, or one's self, to the beard, is considered in the East an intolerable insult.