When someone accepts something "hook, line, and sinker," it means that the person in question has accepted it — that is, an idea or explanation — completely, without any reservations. This idiomatic phrase can be used whenever someone takes information at face value and wholly accepts it. It is often used to refer to someone who is gullible enough to fall for a practical joke or a trick played on them. "Hook, line, and sinker" is a phrase that comes from the pastime of fishing, and it refers to a fish that swallows not just the bait but also everything attached to it.
An idiom is a word or short phrase which often means something quite different from the literal interpretation of the words themselves. These phrases often originate in a very specific industry or arena but come to be used more widely. Their meanings come from the way that people in a certain culture use them and understand them, and they allow speakers to add color and expressiveness to everyday speech. One such idiom that is based on fishing terms is the phrase "hook, line, and sinker."
If this idiom is used, it conveys a complete belief or acceptance of something else. There is no equivocation being shown by a person who is described in this manner. In actuality, a person who merits this phrase takes something in or accepts its trustworthiness or veracity without any hesitation. For example, someone might say, "I thought he might buy just a part of the package, but instead he took the whole thing hook, line, and sinker."
Another way that the phrase is often used is as a means of depicting a certain level of gullibility. If someone is going to blindly accept whatever someone tells them, it opens that person up for all manner of deceit by others. For this reason, this idiomatic expression often comes into play when someone gets tricked or duped by someone else. In this context, consider the sentence, "That salesman was putting on a real show, and he fell for it hook, line, and sinker."
As is the case with many idioms, this phrase comes from a very specific setting and has evolved to where it can be used in many different circumstances. When someone is fishing, he or she must put the bait on a hook, weigh the hook down with a sinker under the water, and attach it to a line leading back up to the fishing pole. Any fish that takes the "hook, line, and sinker," is eating far more than just the bait.