Vertical smile is a slang term for the female genitals. The vagina and vulva can both be referred to as vertical smiles.
The term vertical smile references the way that the labia, or vaginal lips, looks similar to the lips on a face that form a smile. However, unlike the lips on the face, which run horizontally, the lips of the vagina have a vertical orientation. That's why the female genitals are called a vertical smile.
Strictly speaking, any type of female genitalia can be called a vertical smile. However, some people only use the term to describe female genitals with large or prominent vaginal lips.
Vertical smile is seen as a moderately vulgar slang term for the female genitalia. However, it may be seen as more vulgar depending on how it’s used. A factual article with instructions for “keeping your vertical smile fresh and clean” is likely to be seen as witty and playful. Many people also believe references to a woman’s vertical smile opening up or getting wider after sex are distasteful.
