When you point out someone for doing/saying something stupid,meaningless,irrelevant, or something that is just super wrong. Often said with a loud, clear voice.
Guy: Awww look at that cute cat! *points at small dog*
Used as an insult towards someone when they say or do something redundant or idiotic, often confused with the word "Boy." So calling a girl a "Boi" might get them confused what you are talking about.
"I luv bears they r so impotant to mee!!"
"BOI, Shut yo a** up"
GIF Represents what someone does when they call someone a "Boi"