The hard outer covering of an egg; used figuratively in the phrase 'walking on eggshells' to mean being very cautious to avoid causing offense.
From 'egg' (from Old Norse 'eggja') + 'shell' (from Old English 'sciell'). Both elements are ancient, though the compound 'eggshell' solidified in English by the 17th century as a common term.
Eggshells are incredibly sophisticated—they're porous enough to let the chick breathe but strong enough to protect it, and they're made of calcium carbonate (the same mineral in chalk and marble), which is why they're so geometrically perfect and brittle.
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