The unpleasant results of an earlier activity, especially overindulgence in alcohol. For example, A headache is just one of the symptoms of the morning after. This expression originated in the late 1800s as a synonym for a hangover (and was often put as the morning after the night before). By the mid-1900s, however, it was also being used more loosely for the aftereffects of staying up late.
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.