(Salvatore walks in while Don's flexing his muscles)
Salvatore: Look at you, Gidget. Still trying to fill out that bikini?
Don: Summer's coming.
Frances "Gidget" Lawrence was a fictional character popular in the late 50's and early 60's. First introduced in a 1957 novel written by Frederick Kohner, Gidget and her faithful boyfriend Moondoggie were the embodiment of the southern California surf culture of the day. Gidget was first played on screen by Sandra Dee in 1959, but to most Americans, the seminal Gidget was Oscar-winner Sally Field, who played her in the 1965 television sitcom.
The coil device Don was using was designed to develop men's pectoral muscles, but was also believed to increase women's bust size, thus the reference to "filling out that bikini." Even the bikini was still fairly novel at the time, it would reach fad proportion later in 1960 following the release of Brian Hyland's "Itsy-Bitsy Teeny-Weeny Yellow Polka-dot Bikini."