Buttflaps.

Not that I’ve ever heard, but undercheek was a good suggestion. I’d probably go with “base part of the buttocks” unprompted.

Assfault

I think it’s the cleftal horizon

Edit: mb that’s the buttcrack

Assjowl?

I’ve been calling it the “U-bomb” for years

which stands for “UnderBoob Of My Butt”

The underloaf

Butt-scupple

Grips

Crease

Some movie from 80s? 90s? Called it the Inner Diametral Curve

Neuron activation

Assymetry

Under-Bun

some of us call that “polpa da bunda”