Eggshells

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.