Ginger refers to the idea of kissing one's x-'signature' as being medi�val. That may well be so, but - if it is - I'd like to see some evidence that such a thing ever happened. Presumably, some historical document refers to this odd procedure?
And, more to the point, I wonder why there is no reference to the X = kiss concept before the 1760s. The Middle Ages, after all, ended around 1500 AD. I wonder, therefore, why Chaucer, Shakespeare and a host of other early writers never mentioned it.
The explanation I offered earlier follows the word 'probably', but Ginger's suggestion seems like a bit of folk-etymology to me...but what the hey!