Written with as many 'w's as you want, this word is used as a silly exclamation. It can be used as a greeting or as an opinion. However, it can be used at any part of a conversation to break the ice, trivialise proceedings, or add some joviality.
John: "Haha. How are you!?"
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邮件由 email@example.com 发出。我们决不会发送垃圾邮件。