Etymology
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mackerel (n.)

edible fish of the North Atlantic (Scomber scombrus), c. 1300, from Old French maquerel "mackerel" (Modern French maquereau), of unknown origin; perhaps so called from the dark blotches with which the fish is marked, from Latin macula "spot, stain" (see macula). But the word is apparently identical with Old French maquerel "pimp, procurer, broker, agent, intermediary" (itself attested in English in this sense by early 15c.), a word from a Germanic source (compare Middle Dutch makelaer "broker," from Old Frisian mek "marriage," from maken "to make").

The connection would be obscure, but medieval people had imaginative notions about the erotic habits of beasts. The fish approach the shore in shoals in summertime to spawn. Compare ancient Greek aitnaios "an unknown fish celebrated for its marital constancy;" alphēstēs, the wrasse, "a fish with a singular and unsavoury reputation ... a byword for the incontinent and lewd" (both in Thompson, who also notes that the hermaphroditic nature of certain fishes, discovered by modern naturalists, was known to Aristotle). The exclamation holy mackerel is attested from 1876.

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tunny (n.)

large sea-fish of the mackerel order, 1520s, probably from French thon (14c.), from Old Provençal ton and directly from Latin thunnus "a tuna, tunny," from Greek thynnos "a tuna, tunny," possibly with a literal sense of "darter," from thynein "dart along."

 In ancient Greece, the food-fish par excellence, with its own vocabulary of culinary and market terms for the cuts and preparations of it.

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scad (n.)

c. 1600, Cornish name for a type of fish (also known as horse mackerel) abundant on the British coast; a name of uncertain origin, perhaps a variant of shad. OED compares Welsh ysgaden "herrings," Norwegian dialectal skad, Swedish skädde "flounder."

In July, 1834, as Mr. Yarrell informs us, most extraordinary shoals passed up the channel along the coast of Glamorganshire; their passage occupied a week, and they were evidently in pursuit of the fry of the herring. The water appeared one dark mass of fish, and they were caught by cart-loads, and might even be baled out of the water by the hands alone. ["British Fish and Fisheries," 1849] 
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holy (adj.)
Origin and meaning of holy

Old English halig "holy, consecrated, sacred; godly; ecclesiastical," from Proto-Germanic *hailaga- (source also of Old Norse heilagr, Danish hellig, Old Frisian helich "holy," Old Saxon helag, Middle Dutch helich, Old High German heilag, German heilig, Gothic hailags "holy"), from PIE *kailo- "whole, uninjured" (see health). Adopted at conversion for Latin sanctus.

The primary (pre-Christian) meaning is not possible to determine, but probably it was "that must be preserved whole or intact, that cannot be transgressed or violated," and connected with Old English hal (see health) and Old High German heil "health, happiness, good luck" (source of the German salutation Heil). Holy water was in Old English.

Holy is stronger and more absolute than any word of cognate meaning. That which is sacred may derive its sanction from man ; that which is holy has its sanctity directly from God or as connected with him. Hence we speak of the Holy Bible, and the sacred writings of the Hindus. He who is holy is absolutely or essentially free from sin; sacred is not a word of personal character. The opposite of holy is sinful or wicked; that of sacred is secular, profane, or common. [Century Dictionary, 1895]

Holy has been used as an intensifying word from 1837; in expletives since 1880s (such as holy smoke, 1883, holy mackerel, 1876, holy cow, 1914, holy moly etc.), most of them euphemisms for holy Christ or holy Moses. Holy Ghost was in Old English (in Middle English often written as one word). Holy League is used of various European alliances; the Holy Alliance was that formed personally by the sovereigns of Russia, Austria, and Prussia in 1815; it ended in 1830.

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lobster (n.)

large, long-tailed, stalk-eyed, 10-legged marine shellfish (Homarus vulgaris), early Middle English lopster, lopister, from Old English loppestre "lobster," also "locust," a corruption of Latin locusta, lucusta "marine shellfish, lobster;" also "locust, grasshopper," which is of unknown origin. De Vaan writes that "The only word similar in form and meaning is lacerta 'lizard; mackerel', but there is no common preform in sight. ... [T]hey could be cognate words in the language from which Latin borrowed these forms."

The change of Latin -c- to English -p- (and, from late 14c., to -b-) is unexplained; perhaps it is by influence of Old English loppe, lobbe "spider." The ending seems to have been altered by the old fem. agent noun suffix (preserved in Baxter, Webster, etc.; see -ster), which approximated the sound of Latin -sta.

OED says the Latin word originally meant "lobster or some similar crustacean, the application to the locust being suggested by the resemblance in shape." Trilobite fossils in Worcestershire limestone quarries were known colloquially as locusts, which seems to have been the generic word for "unidentified arthropod" (as apple was for "foreign fruit"). Locusta in the sense "lobster" also appears in Old Cornish legast and French langouste (12c.), now "crawfish, crayfish," but in Old French both "lobster" and "locust" (a 13c. psalter has God giving over the crops of Egypt to the langoustes).

As slang for "a British soldier" since 1640s, originally in reference to the jointed armor of the Roundhead cuirassiers, later (1660) to the red coat, the color of a boiled lobster.

Sir William Waller having received from London [in June 1643] a fresh regiment of five hundred horse, under the command of sir Arthur Haslerigge, which were so prodigiously armed that they were called by the other side the regiment of lobsters, because of their bright iron shells with which they were covered, being perfect curasseers. [Lord Clarendon, "History of the Rebellion," 1647]
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