noun

• Any object that, in (literal or figurative) resemblance to the shell of a clam, has a hinge on one edge and two surfaces that close together.
  "Shut your clam-shell!"
• (often attributive) Any object with some other resemblance to either one or both halves of the shell of a clam.
• (often attributive) Any object with some other resemblance to either one or both halves of the shell of a clam.

verb

• (manufacturing) To deform a die in a shape resembling the shell of a clam, as a result of uneven extrusion pressure.
  "Although it produces a nonuniform pressure drop across the die and thus causes nonuniform flow through the die lip, the distribution of the melt does not produce die distortion of ^([sic]) clamshelling."
• (ambitransitive) To open or close by means of a hinge, similar to the way a clamshell opens and closes.
  "To avoid getting clamshelled, first check the seat pins, then climb in before you pin the canopy."