According to The Oxford English Dictionary - sexy - sexually attractive or stimulating.
The thing that gets me with this word is the when it is used in relation to an object. I have heard it used in relation to "Oooo sexy shoes". Does this mean that the look of the shoes makes you fancy them or that they actually turn you on? That a pair of shoes gives you that little frisson in all the right places. Now, I like shoes, but to be honest they just don't do 'it' for me.
Another example of this I've come across is in cookery shows on TV -"a sexy little pudding". I've yet to see a pudding that makes me want to whip off my knickers and jump into bed with it! I've eaten puddings that give me pleasure, that have that wonderful melt in the mouth "ooooooo" moment but I can say, hand on heart, I've yet to find one that has made me orgasmic.
I think that the word 'sexy' should be reserved for proper usage and no longer used for items that cannot turn you on (unless you are a little kinky), otherwise where will it end? "Look at the sexy washing up brush?" or maybe "That is one sexy tin of beans!" or my personal favourite "I can't wait to wear those sexy socks."
Let us resolve to use the word sexy for the right things (imho) - that look that almost turns you inside out, the curve of a well formed buttock or breast, the voice that sends a shiver up your spine and the touch or kiss that melts you the second it connects - those are my idea of sexy.