response to a text/message when nothing else fits/comes to mind. Instead of lamely replying with 'lol' to a message you don't care to answer to or think about, you put 'asdf' just so the person knows you got the message, and because you never thought their message was funny anyway, never mind anything to do with laughing out loud.
(new text message from : Lucifer, 5:00 pm)
ugh, just woke up man, hahahaha I'm so lazy -_-
(reply with :)