Frank Loesser's daughter, Susan Loesser, authored a biography of her father, A Most Remarkable Fella (1993), in which she writes:
"I'd like to get you on a slow boat to China" was a well-known phrase among poker players, referring to a person who lost steadily and handsomely. My father turned it into a romantic song, placing the title in the mainstream of catch-phrases in 1947.
The idea is that a slow boat to China was the longest trip one could imagine. Loesser moved the phrase to a more romantic setting, yet it eventually entered general parlance to mean anything that takes an extremely long time.[3][4]