trefoil
When panties or other similar clothing are skin tight across the ass and pubic area, creating a distinct three leaf clover shape.
If the material stretches straight across between the buttocks, that's not really a trefoil. You need the indentation between them.
Not to be confused with the radiation symbol, which is also known as and based on the trefoil symbol.
See also
* impossible clothes
* wedgie
Aliases: none