expression used to suggest that a situation or an opinion of something has changed for the worse. Normally, it is used to imply that the initial perception of a situation was overy rosy
or optimistic, and after a certain period of time (normally fairly short), the harsh reality
of the situation becomes apparant.
The origin of the term stems from the evolution of the relationship between a married couple during the first couple of years. Normally, during and after the honeymoon a couple is so madly in love that the annoying little habits of each person neither bothers nor irritates their partner. However, after they get back from some wonderful voyage in a tropical clime and the drudgery of everyday life sets back in, they begin to see their partner for who they truly are.
Some examples of situations for which this expression could be used: