“Hook, line, and sinker” means completely. In modern English, it is often used when someone believes a lie, accepts a story too easily, or is fully won over by something.
The idiom comes from fishing. If a fish swallows not just the hook, but also the line and sinker, it has taken everything. That image helps explain the modern meaning: someone has accepted the whole thing without holding back.
Very often, the phrase suggests being fooled. For example, if a person believes an obvious scam, you might say, He believed the scam hook, line, and sinker. It can also be used more broadly for complete acceptance, as in, The audience bought the sales pitch hook, line, and sinker.
- Use it when someone accepts something fully.
- It often carries a warning that the person was too trusting.
- The tone is usually informal.
So the core idea is simple: the person did not just accept part of it. They accepted the whole thing, completely.