“Not by a long chalk” means a wide margin, whether of time, distance, ability, or something else. In a game, ‘a long chalk’ would mean ‘a lot of points.’
“He lost that race by a long chalk. He came in fifteen minutes after everyone else.”
But it’s often used in the negative sense. “Is William going to win the election?” “No, not by a long chalk!”
This phrase dates back to the days when an accounting or a score of almost any kind was marked up on a handy surface using chalk. If you were drinking in a pub, this might be a list of the drinks you owed for. Charles Dickens refers to this in Great Expectations: “There was a bar at the Jolly Bargemen, with some alarmingly long chalk scores in it on the wall at the side of the door, which seemed to me to be never paid off.”
But the expression almost certainly comes from using chalk in pubs to mark the score in a game. Today the practice survives in British pubs mainly in the game of darts. If your opponent has a long chalk, a big score, he is doing well.
A related expression, primarily American, is “not by a long shot.” This idiom is originally military, based on the difficulty of hitting a target at long range, hence an outside chance.
The phrase “long shot” describes something that is highly improbable, that may not come anywhere close to succeeding. Arising sometime in the late 1700s, it refers to the likelihood of hitting a distant target. Today, “long shot” is mainly used in horse racing and means a horse that has little chance of winning.
Do I despair of finding a new phrase to describe every week for the next year? Not by a long chalk!