A clicko is a mis-click or mouse slip in an online game that causes a stone to be played in a different place from where it was intended.
The name clicko derives from a typo on a typewriter.
In online games, most players (though not all) will readily grant an undo for a clicko if it is requested immediately. A prompt request distinguishes the clicko from a thinko whose faults may more slowly impinge on the player's consciousness.
On servers such as KGS, one can set a "mouse anti-slip" preference to avoid clickos. This works by requiring a delay of some number of milliseconds (say 250 which is a quarter second) that the mouse be still before the click is accepted.
This is a modern back-formation in English. The phrase typographical error (inadvertent insertion or deletion or change of a character in text) is often abbreviated as typo. The analogous word was then derived from click.
MarkD: Well, I agree that you should carefully evaluate your move before clicking. But misclicks do happen. I was playing on a notebook some time ago. It was equipped with a touchpad and if you tapped the surface twice, that was interpreted as a left mouse button click.
After playing two wrong stones I found out that this notebook was not really good for playing Go online :) Fortunately my opponent agreed to my undo's, because they were after really obvious misclicks.
Hu: I think the important thing about undos is that an undo for a clicko should be requested immediately. The longer the delay, the more the request will be mistaken for a request for an undo for a thinko.
When playing on a laptop, I alert my opponent to the fact at the beginning. It has occurred to me that some might interpret this as a strategy to prepare to ask for thinko undos, but I think it is fair, and I try to avoid thinko undos and ask for clicko undos immediately.
Velobici: The laptop clicko problem is particularly severe when using laptops with touch pads (skip-pads, skating-rinks) in place of buttons. The laptop that I got for my eldest daughter is so sensitive that I have a hard time using it with a web browser. The pad is so sensitive that I frequently activate it without intending to do so. I never play Go using that thing. It would be a disaster of clicko's. Laptop buttons with some amount of spring force are a much better choice. Unfortunately, these laptops are getting harder to find. IBM seems to one of the few firms still building laptops without skid-pads at this time (20030729)
StormCrow: Most laptops with touch pads will allow you to disable the tap-click function of the touch pad so that only the buttons will actually click.
Malweth: I never use a Laptop for any purpose without a mouse (assuming the laptop is mine and is not being used temporarily or in a remote location). Touchpads annoy the heck out of me because of the double click "feature" (and the slow speed and the poor accuracy). I'd suggest go players with this type of laptop have a full sized (or even a mini) USB mouse with them at all times ^-^