A swing jacket is a short, flared and easy-fitting jacket, usually part of a suit. Swing/swagger jackets originated in the 1930’s but became most popular in the 1950’s. While, it was originally called a swagger jacket, most people use swing today. Currently, sports jackets such as letterman’s jackets are sometimes called swagger jackets.
This is a swing jacket from Second Life.
The A to Z of Fashion Writing 