When people talk about a “slow boat to China,” they usually mean that something is taking a very long time. The term is often used specifically in reference to movements of people or goods. This slang term is most commonly used in American English, and it is especially associated with the film noir era, when gangsters and detectives alike threw this phrase around quite a bit.
The origins of the term appear to lie in the game of poker. Card sharks used to refer to someone who lost slowly and steadily as a “slow boat to China,” referencing the fact that a patient player could “ride” the victim all the way to the bottom of his or her pocket book. The idea was that the winnings were slow and steady, much like a ship on its way to China, making the victim a profitable venture for a poker player.
In 1948, Frank Loesser used the slang term in a slightly different way in his hit song “On a Slow Boat to China.” He used it in a romantic sense, referencing the idea of being so in love with someone that you would want to spend as much time with him or her as possible. Both of these meanings referenced the fact that shipping something to China took a long time, and it would have taken even longer to cross the Pacific on a slow boat.
People continue to use this term to reference long trips, or to suggest that they have a great deal of time to do something. For example, one might say, “we can sit and talk as long as you like, because we might as well be on a slow boat to China.” While the term isn't used as much in the romantic sense anymore, because the famous song has fallen out of favor, various musicians do periodically revive it, causing a corresponding resurgence of the term in popular society.
Some books written or set in the 1940s and 1950s include this slang term, along with a variety of other colorful terms which were popular in this era. Authors and filmmakers like to include the term to lend an air of authenticity to their work, since this piece of American slang was in widespread use during this era.