Author's Note: separate Biblical and Far Eastern encyclopedias coming soon!
A classic one that many children growing up in Russia or Eastern Europe might recall getting scolded for - whistling indoors. Believed to bring poverty, it is thought that by whistling at home, one "whistles away" all of their money. As a child that didn't learn whistling until the ripe age of 11, I spent the first year of knowing how to whistle in fear of getting a solid disciplining for trying to copy the tunes of my favorite songs - be it folk music like Katyusha or Kalinka, to "Crazy Train" or "Paranoid".
Believed to be a remedy for evil eye,
Whenever one makes an overly wishful claim, or on the contrary, a vocal claim of something terrible happening, it's thought that to "unjinx" oneself, or to prevent said thing from occurring, one must knock on a piece of wood. It's believed that by voicing such things, it can cause for them to "manifest" where they might've otherwise never occurred.
This one is very common in North American, Western European, and Anglophone households. Typically done by all individuals "partaking" in the conversation. Opposite of fingers crossed.
The opposite of the above - when wishing for an event to occur, one "crosses their fingers" - undoubtedly a tradition stemming from Christians crossing themselves in brief prayers or requests to God or the Heavens.
Incredibly similar to the example above, done for the same reasons, albeit with the only difference being that in lieu of a piece of wood, one must use their head. Perhaps originating from a common folk motif / humorous belief that the head of a silly fellow is indistinguishable from wood, and given that only a fool could've said something to warrant knocking on wood, their head shall suffice. Typically done by only the individual saying the undesirable phrase.
When speaking evil, or wishing evil on someone, a common response is for the speaker to be scolded with the threat of developing a sore on their tongue; certainly preferable to damnation from their sin of wishin death, or casting an evil eye on someone.
From my personal understanding, this also negates any jinxes or hexes spoken by the individual at whom the phrase is directed, in an effort to prevent excess harm or evil. Perhaps a development of the Biblical "cast of the part which causes you to do evil, to save the whole."
As with many classical superstitions, this one is traditionally used by elders as a means of disciplining or schooling a misbehaving youth; particularly one wishing illness or harm to come to someone.
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