The idiom "big girl's blouse" is a derogatory phrase used to describe a man who is weak or wimpy or who behaves in an effeminate way. The term, a British and Australian slang phrase, is also applied to a man who whines a lot or is especially fussy. Though sometimes used in a pejorative way, it is now most often used in teasing or bantering. Originating in Britain, the phrase is not in common use in the United States. While some phrases once used exclusively to describe men are now applied to women as well, the phrase big girl's blouse continues to be used nearly exclusively to describe males.
As idioms go, this is a fairly new phrase, not coming into use until about the 1960s. The first written reference using the phrase is recorded in 1969 in the sitcom Nearest and Dearest. Some attribute the origins of the phrase to the TV show, though other language experts think it may have been in existence before then and may take its origins from a time when many people line-dried their clothing outdoors. The phrase "flapping like a big girl's blouse" may be the way in which the phrase was originally expressed. This phrase was describing a large piece of clothing flapping in the wind during line-drying and was used to describe a man who became easily frightened.
Rather than big girl's blouse, the phrase is sometimes expressed as big girl's shirt, though it has exactly the same meaning and is used in exactly the same way. It is likely just a variation on the original slang term or is the original slang term. A slang word shirty was popular in the north of England as early as the 1840s and meant someone who was easily irritated. The phrases to get his shirt out and keep your shirt on, both referring to someone preparing to fight, were also in use at this time and were said as a way of telling an angry person to calm down. There is some speculation that shirty, these two phrases, and big girl's shirt are related in origin and entered the common language near the same time.
As with many idioms, the slang phrase big girl's blouse has worked its way into arts and entertainment. An Australian program took the phrase as its title in the 1990s. A sketch comedy show, Big Girl's Blouse's original four episodes and pilot remain popular online and in DVD.