I’ve heard that before, and it cracks me up. That said, girls with “thigh gap” are far, far too skinny for my taste!
This wins it for me.
I will be dropping the term “intermammary sulcus” on my wife before weekend’s end, however.
The following playlet was inspired by this cite and is written by OP.
[CURTAIN]
[Enter ME, a non-descript disheveled man, slightly hungover. IPAD on couch. (IPAD is mute.) ME takes IPAD, fiddles with it. WIKI appears on IPAD, which is visible to all.]
Me
Wait, you’re telling me only humans and great apes have gluteal sulci. That’s it?
Wiki (nods sagely)
Me
You’re full of it. I’m gonna ask the dudes at SD.
[CURTAIN]
OP here. Same story, same attribution, same expression of emotion to SD…
This time, after my dog-park friend had listened to my story/“guess what you call it”/punchline (were her eyes glazing?), she said “Oh, you mean underbutt.”
Not sure if she just invented the term, meaning she never thought of thinking about it (like the rest of the world), or it’s a known term. Certainly makes sense, with “underboob” a quite understandable word used nowadays.
(Side question: when did “sideboob” and “underboob” become a thing enough for the word to enter vocabulary–it’s used certainly by men, but I only noticed it as a word in Internet guy-talk pin-up girl discussing. Does it–the word, not the anatomical feature–have a recent history?)
This time I mentioned a spandex-pants article of clothing I had become aware of. I thought it was called Spanx, but she said that wasn’t it. I didn’t get straight just what Spanx is/are. “Support garments” I think she said. A sexy girdle.
As the they say in Zombie haute couture: plus ça change, plus la même chose.
As a kid we called that area the “sit spots”. This was almost always in connection with a spanking. I.e., it’s where it hurt to sit after one.
Behold: the power of Spanx … http://www.spanx.com/
That’s the spirit! 
I could see ‘gill’ being appropriate as well.
I propose “tuchus sulcus” for the slang term.
We always called it the pusky. 
The taint was the area between the uh… front hole and the back hole; whereas the pusky was perpendicular to that and was basically the width of the area between the legs.
From Viz Comic’s invaluable Profanisaurus:
*1. Toblerone Tunnel
The triangular gap between a woman’s upper thighs, immediately beneath the vulva.
From the evocative shape of said void and the habit of some, who like to slide the eponymous chocolate treat through it.
Woman: Do these jeans make me look fat?
Man: No, but they do accentuate your Toblerone tunnel. *
The plural of that being, I presume, tuchi sulci ?
I read that, and continue to since I’ve never seen it and it is a sound-alike, to “pushky,” which is the Yinglish casual word for the little box religious Jews have to put change in daily for charity.
Heh.
Clean with 50% isopropyl alcohol once a day. That should easily do the trick. If not, then do the alcohol thing and also make sure you do what you can to keep that area dry.