Etymology
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secondary (adj.)

late 14c., secondarie, "belonging to the second class; not chief or principal; second in importance or authority; of a lesser quality or worth; subordinate to something else, depending upon the action of primary qualities," from Old French secondaire and directly from Latin secundarius "pertaining to the second class, inferior," from secundus (see second (adj.)).

Opposed to primary (adj.) or principal (adj.). Of colors, under the old theory, from 1831; in reference to schools or education, from 1809. Of characteristics peculiar to one sex but not necessary for reproduction, from 1780. Related: Secondarily; secondariness.

As a noun from mid-15c. as "thing or place of secondary importance or which is dependent on a primary;" 1590s as "a delegate or deputy." The U.S. football sense of "defensive backfield" is by 1955.

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by-name (n.)
late 14c., "secondary name;" 1570s, "nickname," from by + name (n.).
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by-product (n.)
also byproduct, "secondary or additional product;" 1849, from by + product.
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epiphenomenon (n.)
"secondary symptom," 1706, from epi- + phenomenon. Plural is epiphenomena. Related: Epiphenomenal.
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slug (n.3)
"a hard blow," 1830, dialectal, of uncertain origin; perhaps related to slaughter or perhaps a secondary form of slay.
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slag (v.)
"denigrate," by 1971, from slag (n.) in a secondary sense of "worthless person" (1788). Related: Slagged; slagging.
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sidebar (n.)
"secondary article accompanying a larger one in a newspaper," 1948, from side (adj.) + bar (n.1).
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crankum (n.)

"a 'twist,' an eccentricity," 1822, mock-Latin formation from crank (n.) in the secondary sense.

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Seconal 

1935, U.S. proprietary name (Eli Lilly & Co.), from Secon(dary) Al(lyl)or else from secobarbital (itself short for secondary barbital).

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shucks (interj.)
expression of indifference, 1847, from shuck (n.) in the secondary sense "something valueless" (i.e. not worth shucks, attested in a separate source from 1847).
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