1Any of various plants identified with Greek ἄλυσσον and formerly reputed to cure hydrophobia. In early use: †any of several hoary plants of the family Lamiaceae (Labiatae), perhaps Marrubium alysson (obsolete). In later use: any of various plants belonging to the genus Alyssum, or to the genus Aurinia which was formerly included in it.
2A European weed, Asperugo procumbens (family Boraginaceae), with trailing bristly stems and small blue flowers in the leaf axils.
Late Middle English (in an earlier sense). From mad + wort, apparently after post-classical Latin alyssum.