Working, be back later is commonly found in away messages as a literal term. However, in recent history it has taken on a more generalized meaning along the lines of "I'm way too lazy to change my away message" It's use is usually associated with the very antithesis of work and should not be taken as it's literal meaning. It goes without saying that their exist a myriad of translations for the phrase, some are as follows:
"I don't know how Aim works and cannot turn off my default away message"
"I don't understand why nobody ever talks to me online"