The "Lost Generation" is a term used to refer to the generation, actually a cohort

Cohort (statistics)

In statistics and demography, a cohort is a group of subjects who have shared a particular time together during a particular time span . Cohorts may be tracked over extended periods in a cohort study. The cohort can be modified by censoring, i.e...

, that came of age during World War I. The term was popularized by Ernest Hemingway

Ernest Hemingway

Ernest Miller Hemingway was an American author and journalist. His economic and understated style had a strong influence on 20th-century fiction, while his life of adventure and his public image influenced later generations. Hemingway produced most of his work between the mid-1920s and the...

 who used it as one of two contrasting epigraphs for his novel, The Sun Also Rises

The Sun Also Rises

The Sun Also Rises is a 1926 novel written by American author Ernest Hemingway about a group of American and British expatriates who travel from Paris to the Festival of San Fermín in Pamplona to watch the running of the bulls and the bullfights. An early and enduring modernist novel, it received...

. In that volume Hemingway credits the phrase to Gertrude Stein

Gertrude Stein

Gertrude Stein was an American writer, poet and art collector who spent most of her life in France.-Early life:...

, who was then his mentor and patron.

In A Moveable Feast

A Moveable Feast

A Moveable Feast is a set of memoirs by American author Ernest Hemingway about his years in Paris as part of the American expatriate circle of writers in the 1920s. The book describes Hemingway's apprenticeship as a young writer in Europe during the 1920s with his first wife, Hadley...

, which was published after Hemingway and Stein were both dead and after a literary feud that lasted much of their life, Hemingway reveals that the phrase was actually originated by the garage owner who serviced Stein's car.