What's that sticking out of your pants? Some messages shouldn't be sent.
Let's get straight to the point. Arschfax literally means "butt fax." But don't start conjuring up images of twisting yourself into awkward positions while sitting on the Xerox machine with your pants down. (C'mon, seriously.) We've all got dozens of Arschfaxes at home - most likely in every single pair of pants and underpants we own.
Arschfax refers to that little tag that tells you to wash your silk underwear on the gentle cycle and your cotton blue jeans with dark colors. And it can sometimes get really annoying when it sticks out or pokes - especially when a friend points out that a white tag has been hanging out of your backside all day and you didn't even notice. (All the more reason to cut the little guys off.)
Needless to say, Arsch is not a very polite word in German and should only be used very sparingly, and those who actually say Arschfax tend to be under 20. The slang term was runner-up for Germany's youth word of the year in 2010.