Nomen agentis.
A person whom no prior social contact have been made whit. But the estimate is that the odds are even 1:1 to engage in sexual contact whit this person that same night.
Usually a person of lower to even socioeconomic status than the person using the expression.
"See that girl over there? She's coinflip."
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Friend: "I heard that you got lucky yesterday?"
You: "Lucky? It was a coin flip!"
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