TMI stands for "too much information". This phrase, though not used very often, (as DO NOT WANT is the current Internets standard for extreme displeasure at something) is typically used when someone admits over the Internets via IM something related to their dick or cunt that others would find unpleasant. This also applies when someone admits they view the likes of shotacon or furry porn. This abbreviation is thus a cry of pain over the mental image implanted into the brains of anyone in the conversation.
