The phrase hot dog in a hallway refers to a woman with a loose vagina. The simile is used to describe how the penis fits, or does not fit, in the vagina.
Hot Dog in a Hallway
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Generally, the phrase is used as an insult toward a woman. It is also used as an alternative way to call a woman promiscuous. In the slang phrase, the hot dog represents the penis while the vagina represents the hallway. It is implied that a high volume of sex has stretched the vagina to the width of a hallway.