Throughout the 19th century, the second session of every Congress was held after the most recent election, when some members either had been defeated or did not run for reelection. When they returned for the second session, they were called “lame ducks” (a slang term the British had originally used for someone who went bankrupt). Out of concern that lame ducks were not likely to promote the public interest, the 20th Amendment moved up the opening date of Congress to eliminate most lame duck sessions.
By Donald Ritchie, Our Constitution (Oxford University Press)