eggshell n.

"the body formed in the females of all animals (with the exception of a few of the lowest type) in which by impregnation the development of the fetus takes place," mid-14c., egge, mostly in northern England dialect, from Old Norse egg, from Proto-Germanic *ajja(m) (source also of Old Saxon, Middle Dutch, Dutch, Old High German, German ei, Gothic ada), probably from PIE *owyo-/*oyyo- "egg" (source also of Old Church Slavonic aja, Russian jajco, Breton ui, Welsh wy, Greek ōon, Latin ovum); possibly derived from root *awi- "bird."

This Norse-derived northern word vied in Middle English with native cognates eye, eai, from Old English æg, until finally displacing the others after c. 1500. Caxton (15c.) writes of a merchant (probably a north-country man) in a public house on the Thames who asked for eggs:

And the goode wyf answerde, that she coude speke no frenshe. And the marchaunt was angry, for he also coude speke no frenshe, but wolde have hadde egges, and she understode hym not. 

She did, however, recognize another customer's request for "eyren." Used of persons from c. 1600. Bad egg in the figurative sense is from 1855; bad eggs aren't always obvious to outward view (there was an old proverb, "bad bird, bad egg"). To have egg on (one's) face "look foolish" is attested by 1948.

[Young & Rubincam] realize full well that a crew can sometimes make or break a show. It can do little things to ruin a program or else, by giving it its best, can really get that all-important rating. They are mindful of an emcee of a variety show who already has been tabbed "old egg in your face" because the crew has managed to get him in such awkward positions on the TV screen. [Billboard, March 5, 1949] 

We don't have egg on our face. We have omelet all over our suits. [NBC News anchor Tom Brokaw, some time past 3 a.m. ET on Nov. 8, 2000, after the U.S. television networks called a winner, then retracted the call, in the Bush-Gore presidential election]

 Eggs Benedict is attested by 1898; various Benedicts are cited as the eponym, and the dish itself is said to have originated in the Waldorf-Astoria or Delmonico's, both in New York. The figure of speech represented in to have (or put) all (one's) eggs in one basket "to venture all one has in one speculation or investment" is attested by 1660s. The conundrum of the chicken (or hen) and the egg is attested from 1875.

Bumble, bramble, which came first, sir,

Eggs or chickens? Who can tell?

I'll never believe that the first egg burst, sir,

Before its mother was out of her shell.

[Mary Mapes Dodge, "Rhymes and Jingles," N.Y., 1875]

"hard outer covering," Middle English shel, shelle, from Old English sciell, scill, Anglian scell "seashell; eggshell," which is related to Old English scealu "shell, husk," from Proto-Germanic *skaljo "piece cut off; shell; scale" (source also of West Frisian skyl "peel, rind," Middle Low German schelle "pod, rind, egg shell," Gothic skalja "tile"), with the shared notion of "covering that splits off," from PIE root *skel- (1) "to cut." Italian scaglia "chip" is from Germanic.

Also in late Old English as "a coating or layer." The general sense of "protective outer covering of some invertebrates" is in Middle English (by c. 1400 as "house of a snail;" by 1540s in reference to a tortoise or turtle); the meaning "outer layer of a nut" (or a fruit considered as a nut) is by mid-14c. With notion of "mere exterior," hence "empty or hollow thing" by 1650s. The meaning "hollow framework" is from 1791; that of "structure for a band or orchestra" is attested from 1938. To be out of (one's) shell "emerged into life" is by 1550s.

Military use for "explosive projectile" is by 1640s, first of hand grenades, and originally in reference to the metal case in which the gunpowder and shot were mixed; the notion is of a "hollow object" filled with explosives. Hence shell shock, "traumatic reaction to the stress of battle," recorded by 1915.

Shell game "a swindle" is from 1890, from a version of the three-card game played with a pea and walnut shells.