Thas'si (θασσί, θασσίς; Vulg. Thasi, Hassii), the surname of Simon the son of Mattathias (1 Macc. 2, 3). The derivation of the word is uncertain. Michaelis suggests תִּדשַׁי (Chald.), "the fresh grass springs up," i.e. "the spring is come," in reference to the tranquility first secured during the supremacy of Simon (Grimm, Comment. ad loc.). This seems very farfetched. Winer (Realwb. s.v. "Simon" ) suggests a connection with תָּסִס fervere, as Grotius (ad loc.) seems to have done before him. In Josephus (Ant. 12:6, 1) the surname is written Ματθῆς, v.r. θαδής, θαθής. SEE MACCABEE.