From the name of the queen who fell in love with Tristan in medieval romance; possibly from Celtic, of uncertain meaning, or from Germanic, equivalent to Old High German is (“ice”) + hiltja (“battle”).
English Wiktionary. Available under CC-BY-SA license.
The story of Tristan and Iseult, immensely popular as it was, was too genuine - (shall we say too crude?) - to satisfy the taste of the court for which Chretien was writing.
Unfortunately he himself has been wounded in the fight, and that by a poisoned weapon; and none but the queen of 'Ireland, Isolt, or Iseult, possessed the secret of healing.
There he gives himself out for a minstrel, Tantris, and as such is tended and healed by Queen Iseult and her daughter of the same name.
The seneschal of the court, a coward who has been watching for such an opportunity, cuts off the dragon's head, and, presenting it to the king, claims the reward, much to the dismay of Iseult and her mother.
This is done in the presence of the court; Tristan is pardoned, formally declares his errand, and receives the hand of Iseult for his uncle King Mark.