Jack of All Trades, Master of None

"Jack" was such a common nickname for "John" in medieval England that it became a stand-in term for an anonymous, everyday man, the same instinct behind words like "jack-of-the-clock." "Jack of all trades" as a phrase is recorded from 1612, in Geffray Minshull's account of his time in prison, and it originally carried no insult at all, simply describing someone broadly, usefully skilled. The stinging second half, "master of none," is a much later addition, generally dated to the 18th century, and it's this addition that flipped the phrase from a compliment into a put-down.