tanist
The heir presumptive to the chieftainship or kingship of a Celtic clan in ancient Ireland, Scotland or Mann.
historical
Lover, for her love he prowled with colonel Richard Burke, tanist of his sept, under the walls of Clerkenwell and, crouching, saw a flame of vengeance hurl them upward in the fog.
Where it comes from
From Irish tánaiste (“second-in-command”). Doublet of Tánaiste.
Adapted from the Wiktionary entry tanist (CC BY-SA 4.0).