IDK & IDC CUZ TGIF! "I don't care, and I don't know because thank gawd it's friday!" (Do your friends a solid and take the extra two seconds to actually write in English so they don't have to reply with a "what? IHUA!" and you responding with an additional message, defeating the purpose of abbreviating in the first place.) Otherwise you come across as a tweenage girl who tries to look cool by using as few characters as possible to express a thought.