slang - What is the meaning of all those "w"s in email and SNSs?

You see a lot of w and ww and even www in Twitter and casual chat. What does it mean? I've always thought it was わいわい but never found out. How is it pronounced?

Here's an example from Twitter

なんでじゃろうねwwとりあえず電車なうだから間に合いそう笑

Answer

They derive from 笑う(わらう). They're the Japanese equivalent of "LOL".