Someone asks the speaker for permission to do something, something terrible, apparently, something that sounds terrible to the speaker at least. And the speaker won’t allow it. Not on my watch, says ...

why not 在口语中主要有以下用法。 B：Why not? 可以呀。 A：Let’s eat out tonight. 今晚我们出去吃吧。 B：Yes， why not? 那好啊。 A：I wonder if I could make a living by writing. 我不知靠写作能否维持生计。 A：My girl-friend is in a bad mood. 我的女朋友情绪不好。