Chug is of imitative origin, representing the dull, repeated sound of a laboring engine; the 'gulp down' sense is later American slang. The historical trail runs through chug in onomatopoeic. Its formation is sound-symbolic rather than a regular transparent compound, with the older form echoing the sound named in the history. Used as a verb and noun, chug now means "move with or make a dull repeated engine sound; to gulp down a drink", so the present definition can be read against the older forms and meanings. For dictionary purposes, the important point is the continuity between the historical form and the modern word chug.