A SMAAAASH!!, or a SMAAAASHing hit as described by a Friendly Mole, is the Mother series's equivalent of a critical hit; it is a random occurrence in which a regular attack deals about twice as much damage. This can only occur when using a character's normal attack, such as Bash. Enemies can also SMAAAASH!! when using a standard attack, such as a bite.
In EarthBound Beginnings, SMAAAASH!! hits essentially deal normal damage except that the target's Defense is ignored. A well-known glitch in the Famicom version is that instead of the SMAAAASH!! being its normal color, it might accidentally use an enemy's color palette (i.e. a purple SMAAAASH!! against a Mr. Bat).
The chance both to deal and to prevent a SMAAAASH!! attack in EarthBound Beginnings is affected by the Fight stat.
When a character in the player's party deals such an attack, the screen flashes green twice and "SMAAAASH!!" appears in the text box on the upper side of the screen. If a character from the player's party receives a SMAAAASH!! attack, the screen will instead flash red once.
The chance of dealing a SMAAAASH!! attack in EarthBound is affected by the character's Guts. Surprisingly, the Luck stat has nothing to do with it. Shoot weapons are incapable of SMAAAASH!! hits in the SNES and Super Famicom version which means that Jeff usually can't deal one. However, in what is most likely a bug or oversight, Shoot attacks can deal SMAAAASH!! hits in the Mother 1+2 port.
In Mother 3
SMAAAASH!! attacks in Mother 3 work the same way that they do in EarthBound, except that "SMAAAASH!!" appears above the victim's sprite if dealt to an enemy, and above the character's HP meter if dealt to the player's party. Flint's "Power Smash" Skill is the only non-bashing attack for the playable characters that can SMAAAASH!! enemies; this skill always results in a SMAAAASH!! if it hits.
When a Starman (which appears as an Assist Trophy and as an enemy in Smash Run) is defeated, it is sent flying with a "SMAAAASH!!" graphic appearing. The "SMAAAASH!!" graphic also appears if the Smash Run version of Starman is hit by its reflected PK Beam, which will defeat it in one hit.