The saying "to fall for (someone or something) hook, line and sinker" comes from fishing. A fisherman uses a line (length of nylon or other strong material) to catch fish. Attached to this is the curved metal hook, with the bait on it to attract the fish (which then gets caught on the fish when it tries to eat the fish.) The sinker is a heavy weight which is also attached, to make sure that the line and bait go deep enough into the water to reach the fish.
The saying is usually used to suggest that you have been tricked in some way, typically into believing an untrue story. You might say "He was such a good liar - I fell for his story hook, line and sinker." This would suggest that you were like an exceptionally easy-to-catch fish, that swallowed not only the bait but all the rest of the fishing equipment too.
The saying is usually used to suggest that you have been tricked in some way, typically into believing an untrue story. You might say "He was such a good liar - I fell for his story hook, line and sinker." This would suggest that you were like an exceptionally easy-to-catch fish, that swallowed not only the bait but all the rest of the fishing equipment too.