The episode: “What to do on a Date,” the short before “Swamp Diamonds,” ep. 503
The riff: Insisted by Joel as the short’s wimpy milquetoast hero reluctantly calls a girl for a date.
The explanation: “Forever Plaid” was an off-Broadway musical revue from 1990 that satirized harmonizing pop quartets of the 1950s, in the style of groups like The Four Aces. The gang is making fun of the supposed “everykid” image that our hero possesses–that ubiquitous, clean-shaven, high-waisted look that often appears so silly to modern observers.
Novelty factor: I feel like I’ve heard of the show before at some point, but either way I could guess at the meaning of the riff from the context.