A "bad hair day" is a colloquial term used to describe a day when one's hair is not styled to their liking, often due to factors beyond their control such as weather or humidity. The phrase "bad hair day" can also be used figuratively to refer to a day when things are not going well in general.
The term "bad hair day" has been in use since at least the early 1900s, and it has been referenced in popular culture numerous times, including in songs, movies, and television shows. In recent years, the phrase has become increasingly popular on social media, where people often share photos of their "bad hair days" in a humorous or self-deprecating way.