eggshell (n.)

also egg-shell, "the shell or outside covering of an egg, especially the hard, brittle, calcareous covering of birds' eggs," early 15c., from egg (n.) + shell (n.). It displaced ay-schelle (Old English ægscill), from the native word for "egg." As a color term, from 1894. Emblematic of "thin and delicate" from 1835; the figure of treading on eggshells "to move cautiously" is attested by 1734.