The Gay Divorcee (1934). Betty Grable's (age17) small part in the film.

Let's K-nock K-neez".
Written by Harry Revel (music), Mack Gordon (lyrics).
Song performed by Betty Grable (age 17) as Guest  Performer and Edward Everett Horton.
Dance performed by Betty Grable, Edward Everett Horton.

The jumpsuit that Betty Grable wears in the "Let's Knock Knees" dance number was a garment previously worn by Dolores del Rio in Flying Down to Rio (1933).