What is "phffft"

Phffft is a 1954 black-and-white romantic comedy film starring Judy Holliday, Jack Lemmon, and Jack Carson, and features Kim Novak in a small but notable role. The picture was written by George Axelrod and directed by Mark Robson, and was the second film starring Holliday and Lemmon that year, after It Should Happen to You.