A metaphorical stick one uses to "beat" correct information into an ignorant, incompetent, or slow-learning person (i.e., to help them "get a clue"). A pun on the term "two-by-four," a piece of timber that measures 2 by 4 inches (5 by 10 cm) in height and width. I hate reading comments on online news articles—there are so many stupid people out there that I just want to get a big clue-by-four and knock some sense into them with it!