Butt cleavage panties -- (noun) bikini-style drawers with either a peephole or a lace-up in the back, so named for the "cleavage" view they create
"Interesting But Unnecessary!" says fashion/shopping blog SheFinds.com of the lingerie. I have to respectfully disagree with you on that judgment, SheFinds.com: these panties are cute and totally intriguing! As someone who refuses to wear thongs because they're uncomfortable, there's something comforting about saucily showing just a tiny bit more while still being mostly covered. As is the case with all lingerie, some butt cleavage panties are are tawdry-looking, while others are just begging to be unlaced.
Of course, the concern about so-called butt cleavage is that it'll become as ubiquitous as boob cleavage. I'm reminded of a chat I had with a friend recently -- a straight, red-blooded American male -- who rolled his eyes at a lady we saw on the street, whose mostly-exposed breasts were bouncing around her tiny dress. "Enough already with the cleavage! It's only June!" He moaned, exasperated. "It doesn't need to be everywhere!" I teased him that he's the only dude who feels this way, but he insists lots of guys think it's more special for a girl to only reveal her goodies in the bedroom.
Should my buddy be concerned about the new cleavage on the block? A gal would have to be wearing some very low-riding jeans in order to show off her butt cleavage, so the world is still safe for the delicate eyes...for now.
(Oh, but panty-naming people? This trend seriously needs a sexier name than "butt cleavage.")