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.