The word "dong" can refer to a few different things, but in informal usage, it's often a colloquial term for the male genitalia. It can also refer to a large bell or the sound it makes when struck, especially in expressions of time, like the "dong" of a clock.
The term "dong" as slang for the penis is believed to have originated in the mid-20th century, possibly from the sound it represents or as a playful term. It may also derive from regional dialects or expressions. The origin related to a bell comes from Middle English "dongen," meaning to strike or hit.