When somebody is an athlete during one particular season of the year and gets a six-pack during this time, but after the season ends it fades away due to lack of activity.
Amanda: Hey, didn't Joe have a six-pack during the swim season?

Claire: Yeah, but now he just sits around and plays Modern Warfare 2 all day, he has a seasonal six-pack.
by danisreallycool May 18, 2010
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