NSFW comes from the days of emails (for you younger ones, that's what us old people used before text messages came along) in that if you opened it at the office (again, that's old school, before all office jobs went to India, we called those "white collar" jobs) and your boss walked in behind you, then you might be at risk of losing your job (this is of course, when people actually did work at work. ikr? Back when employees didn't stand around telling their friend what they did on the weekend while the customer waits 20m for a coffee and muffin) because the content is "unprofessional" (yea! it's a real word! look it up!)