A congenital deformity in which the buttocks can open up like the shells of a clam, with anus inside as a clam would be inside clamshells.
He dropped his shorts and then made everyone in the locker-room pass out from the sight of his clamshell anus.
by wiginpen February 16, 2004
An idea that looked good on paper but actually causes more harm to the honest user rather than the intended criminal.

This sealed plastic dome causes many Americans to injure themselves in a magical rage, as there is no way to conventionally open the package with bare hands.

Even with scissors one can find themselves busting a blood vessel trying to free the product imprisoned in the case of failure.
Nathan couldn't get his headphones out of the clamshell packaging, and he resorted to biting and tearing until he split open his lip.
by SkySandvich August 30, 2009
An Oakland clamshell is performed using 3 Necessary Objects, a bulldozer, a large black dildo, a box of Frosted Flakes and your local fat black woman. To proceed with this maneuver you must attach the dildo to the bulldozer (creating the dildozer) you must also make a trail leading to a location near the dildozer (leading your local black woman) and when one appears you must be ready on the dildozer and proceed to ram her anus. Then without letting her escape you must lift her using the dildozer once this maneuver has been successfully completed you must shake her on the dildo causing a violent squirting orgasm. Then this has been completed successfully.
Man 1: I finally did my first Oakland clamshell last night!

Man 2: no way, that maneuver is way to intense.

Man 1: hell yeah I did! It wasn’t easy she put up quite the fight!
by Crusty clamshell February 22, 2019
a sex postion when the girl pulls her feet towards her head and you get her up the arse
Man)Do you wanna FUCK
Woman) Yeah clamshell lets go right hear right now.
by 4twentymasta November 24, 2015