This cartoon by Paul Thomas from the Daily Express relates to the controversial remarks made by double Booker prize-winning author Hilary Mantel about the Duchess of Cambridge, aka Kate Middleton. In a lecture at the British Museum, Ms Mantel described the Duchess as “painfully thin” and a “shop-window mannequin, with no personality of her own, entirely defined by what she wore”. Ouch!
In the cartoon, Mantel is shown at a book signing in a bookshop (do they still exist?). The Queen, Prince Charles, and Prince Harry are standing next to the table where she is signing copies of her latest novel, "Bring Up The Bodies". She asks them, "What shall I put in it?" (i.e., what message should I write). The Queen and Prince Charles reply, "A sock!" Clearly, they are not amused by Ms Mantel's attack on William's wife.
If you tell someone to put a sock in it, you want them to shut up and be quiet because they are annoying you. The Phrase Finder has this to say about the origin of the expression: "The imagery behind the phrase is that putting a sock in whatever was causing the noise would quieten it down. What that thing was isn't known. There are suggestions that this may have been the horn of an early gramophone or, more straightforwardly, the raucous person's mouth".