It's an odd combination of meanings, but, hey, if you're eating the contents of a sinistrous shell then the two meanings do come together.
And possibly not only for the poor shell fish, either.
Word To Use Today: sinistrous. This word comes from the Latin word sinister, which means on the left-hand (or for the Romans unlucky) side.
(If you were trying to tell the future by looking at the flight paths of birds then you really didn't want to see one zooming in from the left).
The word may be connected with the word sinus, which means pocket, Romans keeping their pockets in the left side of their togas.
Going further back, the Proto Indo European word that gave us the word sinister seems to have meant shadow. There's evidence that ancient Europeans worked out their directions starting from the east, where the sun rises. If you do this in the Northern hemisphere this means the light is always on your right and the shadow on your left.
So the sinister side is the dark side, and everything about the connection between left-handedness and bad things falls into place.
(Not that there's any real connection, obviously.)