Two words: Tom Kaulitz.
That's right. Sexy hexy can be used as either an adjective or noun in relation to German band, Tokio Hotel's (now) cornrowed, once-dreadlocked, baggy pants-wearing, wanksta/wigger, older-Kaulitz-twin guitarist, Tom.
It's pretty much in relation to how attractive he is. Especially his jawline. Or, you know, just him. Sometimes about his brother, Bill Kaulitz, but 99.99999 percent of the time, about Tom.
(Can be used for other celebrities, I suppose, but I have never come across for anyone else.)
Santi: Oh no, here we go again.
Lexi: MY SEXY HEXY. AAAHHH.
Lexi: Check out that sexy hexy jawline of Tom's. *drools*
The saying that is now used to describe him is probably closely linked to guitarist Tom Kaulitz of the band Tokio Hotel because of a clip from some official Tokio Hotel video, in which the Kaulitz twins say "sexy sexy".
Except when Tom says it, it comes out, "sexy hexy".
Cue very very very very very loud and high-pitched fangirl squeal.
The saying is used most often to describe either Tom or something about him.
(Video probably be found if one searches "Bill and Tom say sexy, sexy" on Youtube. Although that might just lead you to a bunch of fanvids. If you look long enough, you´ll find it, and if you searched sexy hexy on here, you´re probably obsessed enough to look. Fangirls unite and all that.)
Bill: *higher-pitched, slightly less accent* Sexehh sexxeh.
Lexi and every other TH fangirl ever: OMFFFFFFFGGGGGG AAAHHHHHHHHHHHH I LOVE TOM AAAAAH SEXY HEXY AAAAAAHAHHHH... SQUEEEEEEEEEEEEEEEEEEEE!! *dies.*
Lexi: Hey, have you urbandictionaried sexy hexy yet?
Lexi: WHAT? DON´T YOU LIKE TOM KAULITZ?
Razzle: Uhm...you know the answer to this.
Lexi: BUT HE´S SO SEXY HEXY.