A stuffed shirt refers to someone who is inflexible, a fuddy-duddy, or consumed with an unjustified opinion of great self-importance. The idiom can also mean someone is conservative. The idiom has come into the English language fairly recently, with a few explanations for its origins. The stuffed shirt bears some resemblance to the empty suit, a person whose importance is greatly over exaggerated. Both imply a sort of useless quality to someone and a person who is likely to bore you.
There are two explanations for the origins of term. The first is that anyone who made a scarecrow clearly had to stuff the shirt with hay, newspapers, or old clothes in order to better reproduce a person. The scarecrow is clearly an empty, fake person. He may be puffed up with hay, but is really nothing. The scarecrow does not frighten most humans, and even crows may be unimpressed with this variant of the stuffed shirt.
In the 19th century, many shirts for both men and women were elaborately starched affairs. Merchants might want to show off the shirts in shop windows, and mannequins were not in common supply. Retailers used newspaper or hay to stuff starched shirts, sometimes called shirtwaists, so a potential buyer would get an idea of how the shirt would look when worn. This is the second possible explanation for how the term stuffed shirt arose. Again, it implies an emptiness or a fake instead of a “real” person.
Starching shirts, or creating a stuffed shirt often made the garment appear quite inflexible. Even a starched shirt that is on a hanger retains some of its form. Also the starched or stuffed shirt gives the illusion of being filled with something, when really all it is filled with is air or useless material.
Calling someone a stuffed shirt is an insult. You are either implying that they are really very unimportant, or that they are very inflexible. For example, an adult who constantly corrects the manners of others could be called by this name. Alternately, a stuffed shirt may be someone unwilling to bend on any trivial issue. He or she feels that his or her way of doing things is the one and only correct way, and will be quick to point out or be rude to others who may live their lives according to different standards.