turn tail

to run away in fear (and run is often added although this essentially makes it redundant)

Don't turn tail when you have a huge turnout like that.

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shoot blanks

to be infertile and unlikely to impregnate a woman by having sex with her (from a term used to describe the firing a gun loaded with bullets designed not do harm to the target)

She never realized he was shooting blanks until he hesitated about buying condoms. (example of shoot blanks)

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