thallus n.

fem. proper name, from Latinized form of Greek Thaleia, "the joyful Muse," presiding over comedy and idyllic poetry, literally "the blooming one," fem. proper name from adjective meaning "blooming, luxuriant, bounteous," from thallein "to bloom," related to thalia "abundance," thallos "young shoot" (see thallus). Also the name of one of the three Graces, patroness of festive meetings.

rare metallic element, 1861, Modern Latin, from Greek thallos "young shoot, green branch" (see thallus) + element name ending -ium. So called by its discoverer, Sir William Crookes (1832-1919), from the green line in its spectrum by which he detected it. Related: Thallic.