tribe (n.)

mid-13c., "one of the twelve divisions of the ancient Hebrews," from Old French tribu or directly from Latin tribus "one of the three political/ethnic divisions of the original Roman state" (Tites, Ramnes, and Luceres, corresponding, perhaps, to the Latins, Sabines, and Etruscans), a word of unknown origin. The first element generally is held to be tri- "three," and the whole to represent a PIE indication of "tripartite division" (de Vaan).

The extension to modern ethnic groups or races of people is from 1590s, specifically "a division of a barbarous race of people, usually distinguishable in some way from their congeners, united into a community under a recognized head or chief" [Century Dictionary, 1891], but colloquially of any aggregate of individuals of a kind.

In reference to the Twelve Tribes of Israel, the oldest sense in English and almost the only one in Middle English, the Latin word is a specialized Church sense, translating Greek phylē "race or tribe of men, body of men united by ties of blood and descent, a clan" (see phylo-), and English tribe also was used from 17c. of the phylai of the Athenians.