also diehard, 1844, in reference to the 57th Regiment of Foot in the British Army, from the verbal phrase die hard "suffer, struggle, or resist in dying," 1784; see die (v.) + hard (adv.). As an adjective, attested from 1871. The  brand of automobile battery, spelled DieHard, was introduced by Sears in 1967.

also hardboiled, 1723 in reference to eggs, "cooked so long as to be solid," from hard (adj.) + past tense of boil. In transferred sense "severe, tough," from 1886.

"tough, uncompromising," 1961, from hard (adj.) + ass (n.2). Probably originally military. As a noun, "tough, uncompromising person," from 1967. Old Hard Ass is said to have been a nickname of Gen. George A. Custer (1839-1876) among his cavalry troops because of his seeming tirelessness in the saddle.

also eggbeater, 1828, "instrument having a piece to be twirled by the hand, for use in whipping eggs," from egg (n.) + beater. Slang sense of "helicopter" is by 1937 from notion of whirling rotation.

"sand-glass used for determining the time in boiling eggs," 1873, from egg (n.) + timer.

also eggnog, "sweet, rich, and stimulating cold drink made of eggs, milk, sugar, and spirits," c. 1775, American English, from egg (n.) + nog "strong ale." Old recipes for the drink could be made with weak alcoholic beverages like beer or wine in lieu of the milk.

… And Bryan O'Bluster made love to egg nog, 

And pester’d the ladies to taste of his grog; 

Without it (said Bryan) I never can dine, 

’Tis better by far than your balderdash wine,

It braces the nerves and it strengthens the brain,

A world and no grog is a prison of pain,

And MAN the most wretched of all that are found 

To creep in the dust, or to move on the ground!

It is, of all physic, the best I have seen 

To keep out the cold, and to cut up the spleen —

Here madam — miss Cynthia — ’tis good — you’ll confess — 

Now taste — and you’ll wish you had been in my mess.

[Philip Freneau, The Passage to Burlington, ca. 1790]

also hardhearted, "obdurate, unfeeling," c. 1200, heard-iheorted," from hard (adj.) + -hearted. Sometimes in Middle English also meaning "bold, courageous" (c. 1400). Related: Hard-heartedly; hard-heartedness. In late Old English and early Middle English, hard-heort meant both "hard-hearted" (adj.) and "hard-hearted person" (n.).

1838 of Baptists (figuratively); 1798 of clams; see hard (adj.) + shell (n.). Hard-shelled is from 1610s.

"penile erection," 1922, earlier as an adjective (1893), from hard + on.

"stubborn," 1927, from hard (adj.) + nose (n.). Earlier of bullets or shells with hard tips, and of dogs that had difficulty following a scent. Not in common use before 1950s, when it begins to be applied to tough or relentless characters generally (Damon Runyon characters, U.S. Marines, Princeton professors, etc.). Soft-nosed seems to have been used only of bullets.