The gap! Ah that old taunt from HS days/ can’t help I have wide hips, even in maturity with added pounds to my frame it remains.
A word I just learned a few weeks ago from a book which I assume is by a British author
Women equipped with thigh gaps are far, far more desirable than the other kind. At least for a hefty subset of male observers.
I would not ordinarily bring this up as these sorts of comments are frowned on a bit nowadays around here. But you sounded sad about your configuration and I mean to brighten your day.
I have heard the phrase used in a bantering way to refer to a glimpse of “builder’s crack” (the French equivalent of which, I have recently learned, is “le sourire du plombier” - “plumber’s smile”)
They use the expression in English on subways in Japan, as well.
I wasted all these years not being offended.
I’ve always heard that referred to as “plumbers crack”.
Putting the English and French together I wonder if that’s the origin of “crack a smile”? 
I can no longer hear about airline safety demos without chuckling.
That reminds me of one of Lily Savage’s greater NSFW rants (from about 54sec):
