A John is a blanket term used to "justify" an event occuring in a game, such as losing, missing an advantage or narrowly winning match, among others.
The term John, according to a Californian Smash Bros. player 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. As a result of the escalating use of "Johns" in the Smash community, the phrase "No Johns" has received equal popularity, often as a retort to players who use "Johns". However the term was later used for other games besides Smash Bros.