This idiom, or figure of speech, can be broken into two obvious parts: “jack of all trades” and “master of none”.
The real “meat and potatoes,”* or substance, of this phrase as a whole is “jack of all trades,” so we’ll start there.
“Jack of all trades” is said to have evolved from the Elizabethan (1500s) English phrase “Johnny do-it-all”. The earliest written reference to “Jack of all trades” is in Essayes and Characters of a Prison by Geffray Mynshul in 1618. Though “Johnny do-it-all” tended to have a negative connotation in its day, “Jack of all trades” doesn’t have to be negative. It is used to refer to someone who involves themselves in many different kinds of work or hobbies. With its original negative connotation, the phrase’s original expansion was “Jack of all Trades is of no Trade” (Thomas Fuller, Gnomologia, 1732).
Then comes “master of none” in the turn of the 19th century: “’How comes it that I am so unlucky?’ ‘Jack of all trades, and master of none.’” (Maria Edgeworth, Popular Tales: Will, 1800). With this addition, the full phrase refers to someone who is competent in many skills, but does not excel in any of them and is just a variation of “… is of no trade.”
Without the phrase “master of none” attached, the speakers’ intentions are dependent on the context of the conversation, but at least now, to be called a “jack of all trades” does not mean that you are lacking in skill, it simply means that you are someone who is well versed and can do many things!
*The “meat and potatoes” of something simply means the basics and most essential part of something.
Dictionary of Idioms and Their Origins, Linda and Roger Flavell