The Thundercloud is an item that was introduced in Mario Kart Wii, and can be obtained from item boxes scattered throughout tracks. It was one of the three items that was debuted in this game but has not made a reappearance since, the other two being the Mega Mushroom and POW Block.
When obtained by the driver, it delivers them a brief period of continuous boosting. However, it must be transferred onto another driver within the short time limit by colliding onto them, as when it disappears it will electrocute and shrink the unfortunate driver, much like the effects of standard lightning. Therefore, the item pretty much serves as a double-edged sword - it can benefit the driver by harming an opponent, or harm the driver themselves.
The Thundercloud is a bluish silver, smiling cloud with a yellow bolt of lightning above it, hence the name "thunder".