piss on my chips and tell me it's vinegar.

This is a 'mangled metaphor'. It mixes up 3 idioms:

Piss on my leg and tell me it's raining
Piss on one's chips
Piss and vinegar

To deliberately and painstakingly go out of one's way to ruin a friend's chances of success. Usually done jokingly with reference to the opposite sex.

No way do I want you as my wingman. You would piss on my chips and tell me it's vinegar.

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