From a possessed form of Proto-Semitic *raʔš- 'head'

Proto-Kartvelian *taw- 'head'

Klimov, G. A. (1998), Etymological Dictionary of the Kartvelian Languages (Trends in linguistics. Documentation; 16), New York, Berlin: Mouton de Gruyter, page 66 https://archive.org/stream/KlimovEtymological/Klimov#page/n82/mode/1up 
Fähnrich, Heinz; Sarǯvelaʒe, Zurab (2000), Kartvelur enata eṭimologiuri leksiḳoni [Etymological Dictionary of the Kartvelian Languages] (in Georgian), 2nd edition, Tbilisi: University Press, pages 226–227 https://archive.org/stream/FahnrichSarjveladze/Fahnrich#page/n225 
Fähnrich, Heinz (2007), Kartwelisches Etymologisches Wörterbuch [Kartvelian Etymological Dictionary] (Handbuch der Orientalistik; VIII.18) (in German), Leiden, Boston: Brill, page 186 https://archive.org/stream/Fahnrich_2007/Fahnrich%202007#page/n185/mode/2up 

Kajaia, Otar (2005), Megrul-kartuli leksiḳoni [Mingrelian–Georgian Dictionary], online version prepared by Joost Gippert, Frankfurt am Main, published 2001–2004, page 592 https://archive.org/stream/kajaia#page/n591/mode/1up