For the condition that occurs causing characters to shake back and forth while immobile, see Paralysis.
Stun is an effect in Super Smash Flash 2 that leaves the player unable to do an action for several moments. Stunned character notably look dazed or confused, some even hardly pant as well. As with other status, button mashing reduces the stunning time.
↑ 1.01.1These elements currently stun opponents as of the most recent version of the Beta but will put opponents to sleep instead in a future update.
Bandana Dee, Marth, and Sandbag are the only characters that use their sleep animations whenever they get stunned but those are otherwise treated as their actual stunned animations.
Moreover, Waluigi, Kirby, Ness, and Sora is also the only characters that produces vocal noises when they're stunned. Conversely, Black Mage produces a high pitch sound effect when he is stunned as well.
Goku's stun animation is a reference to the heart disease he contracted via a virus during the "Androids saga" arc in the Dragon Ball manga and the anime Dragon Ball Z.