A John is an excuse for losing, missing a recovery, being hit by an attack, or any number of unfavorable events in Smash. It is a statement or action that detracts from the validity or quality of a competition (or its implied significance), irrespective of truth.
The term John, according to California smasher HugS, comes from a Smash player in Texas (named John) who would consistently invent excuses every time he lost. The term "Johning" became commonplace in the community, as others grew tired of his excuses, thus coining the term.
In Europe, the term "Yuna" can be used interchangeably with "John", here referring to the Smasher, Yuna, who has the same habit. It's most commonly said completely in Swedish, "Inga Yunas".
No Johns
The phrase "No Johns" means "no excuses". It is frequently used as a retort when another player uses an excuse.