The word 'bachelor' comes ultimately from the Latin 'baccalaris', as Clanad says, which related to a subdivision of land. The person to whom it belonged probably worked for a larger land-owner...a kind of share-cropper, it would seem.
In 13th century England, the word meant a junior knight, not old enough or rich enough to fly his own banner. Notice the 'subordinate' concept yet again. By the 14th century, it had come to be applied to a first degree, again to indicate that the holder had not yet attained a Master's position/degree. Basically, then, the word has just meant 'junior' for centuries. (Incidentally, it was not applied to an unmarried man until later than its application to the first degree.)