should do, but don't; there's no way ~
Expresses strong denial or refusal, meaning 'there's no way (I) would do ~' or '(I) certainly won't do ~'.
This pattern is a colloquial, emphatic negative formed by inserting や between the ます-stem and しない. It conveys the speaker's strong determination not to do something, often with a tone of defiance or disbelief. It is used in spoken Japanese, especially in casual or rough speech, and less so in polite conversation or writing.
When you use やしない, you are not just saying 'I won't do it', but rather 'There's no way I'd do it' – it implies that the action is unthinkable or that the speaker firmly rejects it. It can also be used for third persons, expressing that someone absolutely will not do something.
Because it is informal and strong, it is not suitable for formal situations. It appears in dialogues, manga, and casual speech, and may sound blunt if used with people you should be polite to.