I would appreciate an answer to this question.
A few years ago, I realized that the word "jack" is used frequently in the English language. I have heard you say that the origin of "jack" is unknown. So I have developed my own theory and I would like you to tell me if its plausible. I first realized the prominence of jack because of how often it is used in children's tales and fables, for example: Jack and the beanstalk, Jack Sprat, The house that Jack built, Jack be nimble and etc. A friend and I listed about 90 words that contain the word Jack and we realized it is used in every part of our language, plant, animal, food, tools, jobs, profanity as well as titles of books, movies and songs. My theory is that before the human species could talk the most common sound that was used was "jk" because that sound takes the least amount of facial muscles and tongue movement. As language developed that sound was extended to "jack" and continued to be used to describe the environment. This propensity continues today. Does this sound plausible? Is there any other information you can give me on this issue?