"hard coat of the eyeball," 1886, medical Latin, from Greek sklēra (menix) "the hard (membrane)," fem. of sklēros "hard" (see sclero-).

Proto-Indo-European root meaning "to cook, ripen." 

It forms all or part of: apricot; biscuit; charcuterie; concoct; concoction; cook; cuisine; culinary; decoct; decoction; drupe; dyspepsia; dyspeptic; eupeptic; kiln; kitchen; peptic; peptide; peptone; precocious; pumpkin; ricotta; terra-cotta.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit pakvah "cooked, ripe;" Avestan -paka- "cooked;" Greek peptein "to cook, ripen, digest," pepon "ripe;" Latin coquere "to cook, prepare food, ripen, digest, turn over in the mind," Oscan popina "kitchen;" Lithuanian kepti "to bake, roast;" Old Church Slavonic pecenu "roasted;" Welsh poeth "cooked, baked, hot."

c. 1200, transitive, "make (something) hard," from hard (adj.) + -en (1). Intransitive meaning "to become hard" is late 14c. The earlier verb was simply hard, from Old English heardian. Related: Hardened; hardening.

c. 1200, "wise, prudent," from well (adv.) + done. Meaning "thoroughly cooked," in reference to meat, is attested from 1747. Well done! as an exclamation of approval is recorded from mid-15c.

1831, from Greek eupeptos "having good digestion," from eu- "well, good" (see eu-) + peptos "cooked, digested," verbal adjective of peptein "to cook" (from PIE root *pekw- "to cook, ripen").

Spanish dish of rice with chicken and other meat, seafood, vegetables, etc., cooked together in a large, flat pan, 1892, from Catalan paella, from Old French paele "cooking or frying pan" (Modern French poêle), from Latin patella "small pan, little dish, platter," diminutive of patina "broad shallow pan, stew-pan" (see pan (n.)). So called for the pan in which it is cooked.

c. 1400, obduracioun, "hard-heartedness; defiant impenitence," from Late Latin obdurationem (nominative obduratio) "a hardening," noun of state from past-participle stem of Latin obdurare "harden, render hard; be hard or hardened" (see obdurate).

in biology, "sac or pouch serving as a receptacle for eggs," 1875; see oo- "egg" + cyst.

1846, "a hard hit," from slog (v.). Sense of "spell of hard work" is from 1888.

"native or permanent resident of London," specifically the City of London, more precisely one born or living "within the sound of Bow-Bell" (see Bow bells); c. 1600, usually said to be from Middle English cokenei, cokeney "spoiled child, milksop" (late 14c.), originally cokene-ey "cock's egg" (mid-14c.). The most likely disentangling of the etymology is to start from Old English cocena "cock's egg" -- genitive plural of coc "cock" + æg "egg" -- medieval term for "runt of a clutch" (as though "egg laid by a cock"), extended derisively c. 1520s to "town dweller," gradually narrowing thereafter to residents of a particular neighborhood in the East End of London. Liberman, however, disagrees:

Cockney, 'cock's egg,' a rare and seemingly obsolete word in Middle English, was, in all likelihood, not the etymon of ME cokeney 'milksop, simpleton; effeminate man; Londoner,' which is rather a reshaping of [Old French] acoquiné 'spoiled' (participle). However, this derivation poses some phonetic problems that have not been resolved.

The characteristic accent so called from 1890, but the speech peculiarities were noted from 17c. As an adjective in this sense, from 1630s. Related: Cockneydom; Cockneyish.