The basic idea is that Israel is derived from the root yashar by means of the name yeshurun (Deut 32:15). Jewish mystics have always interpreted Israel as meaning 'straight (= yashar) to God.' Like all Hebrew words yashar can be translated by many different Greek words depending on the context. While chrestos (= χρηστὸς) is used in Proverbs 2:21 LXX the more frequent terminology is εὐθύς, a term which appears especially frequently in the Gospel of Mark undoubtedly because yashar was so important to the early Christian identity associated with the text.
I am one of the few who believe that a Hebrew gospel - perhaps one in Aramaic- was behind our present Greek editions. It is possible in my mind that the Marcionite text may well have been written originally in a Semitic language (the Marcionites certainly survived in great numbers as a Syriac speaking community). Let's notice the important of the word peshat in the story of the man with the withered hand:
And he beheld them with indignation, while it grieved him on account of the hardness of their hearts. And he said to the man, Stretch out thy hand. And he stretched (it); and his hand straightened (upeshat wetubh leh yedha).This narrative has always puzzled me. Why would the gospel author spend so much time telling the story of Jesus 'straightening out someone's hand'? Surely resurrecting the dead or healing the sick are more impressive miracles. The answer must be that the idea of 'straightening' someone has symbolic significance. My guess is that it goes back to understanding that the individual is preparing to become a yasharim. Already Ibn Ezra takes peshat to be a synonym for yashar (Comm on Numbers 22:28). I think Mark had the same idea with regards to this story.