user12686: Where did this rule originate? Why does it exist?
I just recently learned about it, and in my casual coffeeshop chess games, it has never been mentioned or enforced.

user2929: If you're allowed to touch a piece, or touch a square with a piece, without having to commit to the move, you could pretend to move, look at your opponent's reaction, and then retract the move if you see a reaction indicating you've just made a blunder.
Chess is not Poker, and the game should be decided on skills over the chessboard, not on psychological tricks or how good of a poker face you can make.

user7297: If you're allowed to touch a piece, you can also pick it up and land it on another square, look how the new position would look like, and return it to its original square if you don't like the new position. Repeat this over and over again, and it becomes very annoying for your opponent.
This explains why the rule might exist, but not since when. I do not know the answer to that question.

12686: <p>Where did this rule originate? Why does it exist?</p>

<p>I just recently learned about it, and in my casual coffeeshop chess games, it has never been mentioned or enforced.</p>
 2929: <p>If you're allowed to touch a piece, or touch a square with a piece, without having to commit to the move, you could <em>pretend</em> to move, look at your opponent's reaction, and then retract the move if you see a reaction indicating you've just made a blunder.</p>
<p>Chess is not Poker, and the game should be decided on skills over the chessboard, not on psychological tricks or how good of a poker face you can make.</p>
 7297: <p>If you're allowed to touch a piece, you can also pick it up and land it on another square, look how the new position would look like, and return it to its original square if you don't like the new position. Repeat this over and over again, and it becomes very annoying for your opponent.</p>
<p><sub>This explains <em>why</em> the rule might exist, but not since <em>when</em>. I do not know the answer to that question.</sub></p>
