A woman who upon first meeting comes across as strikingly attractive but, who after time gradually loses her appeal.
It derives from the overwhemling sensation you get when you first step into a hot tub. Once acclimated, however, you want to kick yourself for ever making such a big deal in the first place.
Mikey: I can't believe I was so obsessed with Melissa. What was I thinking?
E: It's not your fault - you fell for a hot tub. It was just a matter of time before you came to your senses.