a fart's chance in a windtunnel
A chance so tiny, it’s like a fart being blown out of a windtunnel and then laughed at by a crowd of engineers.
My chances of getting a raise are a fart’s chance in a windtunnel.
She had a fart’s chance in a windtunnel of making it through the interview.
That plan had a fart’s chance in a windtunnel of working out.