Shave-and-a-haircut

Shave-and-a-haircut referred to a short musical tune, ending in two final notes. In 4 BBY, Lando Calrissian tapped the beginning of shave-and-a-haircut onto the deck plating covering the hidden floor compartments in the Millennium Falcon, in which the Imperial narcotics officer Waywa Fybot was surreptitiously occupying at the time. Fybot, recognizing the tune from Calrissian, responded by tapping the tune's final two notes, thus confirming he was inside the compartment.

Behind the scenes
Shave-and-a-haircut is a reference to the real-world "Shave and a Haircut," a seven-note musical couplet recognizable in popular culture.