participating with a certain degree of outstanditude

On ma'Gawd - outstanditude? Faldage strikes again. Great word.
My understanding was that you got a "letter" in each sport in which you played with a Varsty team and that therefore, as said previously, you could easily get six letters in four years by participation in different seasonal sports.
How-sum-ever I do believe that the term, as used for the man in question "Six Letter Man" is perjorative in the sense that a six-letter man would be a "bit above you all" perhaps a "know-it-all" and not one to let the opportunity pass to let you know he knows it all ... a real pain in the asterisk.