conc▪ According to etymonline.com, Ar tunn (on which Engl tuna seems to be based) is a borrowing, probably made in Spain, from Lat thynnus, thunnus ‘tuna, tunny’, which is from Grk thýnnos ‘id.’, possibly with a literal sense of ‘darter’, from thýnein ‘to dart along’. For a possible relation between the Grk etymon and Ar tinnīn ‘dragon’, see ↗tinnīn.
▪ Rolland2014 summarizes: perhaps from Grk thýnnos ‘id.’, unless it is the other way round or both stem from the “Mediterranean word” mentioned by Chantraine1977.