Etymology
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dog days (n.)

"period of dry, hot weather at the height of summer," 1530s, from Latin dies caniculares, the idea, though not the phrase, from Greek; so called because they occur around the time of the heliacal rising of Sirius, the Dog Star (kyōn seirios). Noted as the hottest and most unwholesome time of the year; often reckoned as July 3 to August 11, but variously calculated, depending on latitude and on whether the greater Dog-star (Sirius) or the lesser one (Procyon) is reckoned.

The heliacal rising of Sirius has shifted down the calendar with the precession of the equinoxes; in ancient Egypt c. 3000 B.C.E. it coincided with the summer solstice, which also was the new year and the beginning of the inundation of the Nile. The "dog" association apparently began here (the star's hieroglyph was a dog), but the reasons for it are now obscure.

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sea-dog (n.)
1590s, "harbor seal," from sea + dog (n.). Also "pirate" (1650s). Meaning "old seaman, sailor who has been long afloat" is attested from 1840.
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moon-dog (n.)

dog who bays at the moon, 1660s, from moon (n.) + dog (n.). Earlier in same sense was mooner (1570s).

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bird-dog (n.)
"dog used in hunting game birds," 1832, from bird (n.1) + dog (n.). Hence the verb (1941) meaning "to follow closely."
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dog-tag (n.)

"soldier's identity disk," 1918, U.S. slang, from dog (n.) + tag (n.1). So called perhaps from resemblance to the identification/license tag on dog collars.

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dog-ear (v.)

also dogs-ear, "to use a book so as to leave the corners of the leaves soiled and curled over" (like the ears of a dog), 1650s. Dog's ear (n.) is by 1725. Dog-eared in the general or extended sense of "worn, unkempt" is by 1894.

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dog-trot (n.)

"a gentle trot, like that of a dog," mid-15c.

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dog-gone (adj.)

also doggone, colloquial minced epithet, by 1849, Western American English, a "fantastic perversion of god-damned" [Weekley]. But Mencken favors the theory that it is "a blend form of dog on it; in fact it is still often used with it following. It is thus a brother to the old English phrase, 'a pox upon it,' but is considerably more decorous." Dog on it was the usual early spelling, so it was perhaps at least felt as such by those using it.

But there are many examples of similar words serving as euphemistic perversions of God: Compare dod for "God" in many oaths (late 17c. through 19c.); dodgasted (probably "God-blasted," in use late 19c., early 20c.); dod-rot (1842).

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dog-paddle (n.)

"type of swimming stroke resembling a dog's swimming," 1860, from dog (n.) + paddle

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