Emily Dickinson Lexicon

TAN'IST, n. [Gaelic, tanaiste, a lord, the governor of a country; in Ireland, the heir apparent of a prince; probably from tan, a region or territory, or from the Gr. δυναστης, a lord, which is from δυναμαι, to be powerful or able, the root of the Gaelic duine, a man. But both may be of one family, the root tan, ten, Gr. τεινω, L. teneo, W. tannu, to stretch, strain or hold.]

Among the descendants of the Celts in Ireland, a lord, or the proprietor of a tract of land; a governor or captain. This office or rank was elective, and often obtained by purchase or bribery. Davies.