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A girl who acts bisexual, making out or engaging in sexual acts with other women, but only in the context of a male-centered crowd and usually when large amounts of alcohol have been consumed. Such a person, though it may appear otherwise on the surface, only takes heterosexual relationships seriously.
"Dude, did you see Jessica making out with that girl at the party? I didn't know she was a lesbian!"

"Nah, man, she's only party bi."

by High Priesthisss September 25, 2013
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A person who is typically and consciously heterosexual, until under the influence of drugs or alcohol in a social gathering. If aroused under such influence, this person ceases to care about their 'sober' identified sexuality, likely only initially identifying as 'straight' due to social grooming and/or having a tendency to favor the opposite sex.
"John is straight, right?"
"He says he is, but once he starts drinking he hits on guys too."
"So John's a Party Bi?"
"At the very least."
by SpaceAce21 March 13, 2017
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