The term 'Jack of all trades, master of NONE' is an old proverb that has been used for centuries to describe someone who is competent in many different areas but does not specialize in any one particular field. The crossword solution for the phrase 'Jack of all trades, master of ___' is 'NONE', which is a four-letter word. This phrase is often used to describe someone who is knowledgeable in many areas but lacks expertise in any one area.