From the Yiddish Dictionary:
Kinehora n : A curse in reverse. A colleague says with best intentions; "Looks like you're going to get a promotion, Jack." Kinehora! You quickly cover his mouth, for to utter such a thing is to ensure it will never happen.

The evil eye, ’eyn hora, is pretty cross cultural. The phrase in Yiddish is keyn ’eyn-hora 'no evil-eye; knock on wood'. In Italian the sign to make to counter the malocchio is the cornuto 'the horns' (index and pinkie stretched out with middle and ring finger curled under covered by thumb) or the fica 'the fig' (thumb between the index and middle finger). The evil eye can be the envious glance of somebody (usually considered a witch). Bad mojo. Ptui, ptui, ptui. &c.

And likewise, kinehora is to the Jewish what "knock on wood" is to... who started that one anyway?

I checked Robert Hendrickson's Encyclopedia of Word and Phrase Origins, and he says "the superstition is an old one and has many possible explanations, none sure. It may be of pagan origin, deriving from the practice of rapping on trees to ask protection from friendly spirits who were believed to reside inside. Or it could be a Christian superstision similar to touching wodden cricifixes or rosary beads."

Here's what Michael Quinion has to say:

http://www.quinion.com/words/qa/qa-tou1.htm

I always assumed it to be pre-Christian, too.

When I was little, growing up in Poland, it was customary to knock on wood whenever you did something bad (like swearing). That was in reference to confession where afterwards the priest tells you to go in peace and knocks on the (wooden) confessional three times.