Daasyochid: The word ‘bochur’ may come from the root ‘choice ones’ but that has nothing to do with yeshiva at all. The word bochur appears many times in shas and meforshim and it simply is a reference to unmarried males.
That said I would agree that colloquially the word ‘bochur’ has become a reference to yeshiva bochurim. After all we don’t talk about ‘university bochurim’.