Etymology
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frog (n.1)

Old English frogga "frog," a diminutive of frosc, forsc, frox "frog," a common Germanic word but with different formations that are difficult to explain (cognates: Old Norse froskr, Middle Dutch vorsc, German Frosch "frog"), probably literally "hopper" (if from PIE root *preu- "to hop," source also of Sanskrit provate "hops," Russian prygat "to hop, jump"). Watkins calls the Old English -gga an "obscure expressive suffix."

The Latin word for it (rana) is imitative of croaking. Also in Middle English as frok, vrogge, frugge, and with sometimes plural form froggen. Collateral Middle English forms frude, froud are from Old Norse frauðr "frog," and native alternative form frosk "frog" survived in English dialects into the 19c.

I always eat fricasseed frogs regretfully; they remind one so much of miniature human thighs, and make one feel cannibalistic and horrid .... [H. Ellen Browning, "A Girl's Wanderings in Hungary," 1896]

As a British derogatory term for "Frenchman," 1778 (short for frog-eater), but before that (1650s) it meant "Dutch" (from frog-land "marshy land," in reference to their country).

The principal inn on the island of Texel is called the Golden Frog, (de Goude kikker). We may wonder that there are not more examples of this sign in Holland, for there are, without doubt, as many frogs in that country as there are Dutchmen ; and even unto this day it is a mooted point, which of the two nations has more right to the possession of the country ; both however are of a pacific disposition, so that they live on in a perfect entente cordiale. [Larwood and Hotten, "The History of Signboards," 1866]

To have a frog in the throat "be hoarse" is from 1892, from frog as a name for a lump or swelling in the mouth (1650s) or throat infections causing a croaking sound.

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frog (n.2)

type of fastening for clothing, 1719, originally a belt loop for carrying a weapon, of unknown origin; perhaps from Portuguese froco, from Latin floccus "tuft of wool," a word of unknown etymology.

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frog-march (n.)
also frog's march, 1871, a term that originated among London police and referred to their method of moving "a drunken or refractory prisoner" by carrying him face-down between four people, each holding a limb; the connection with frog (n.1) perhaps being the notion of going along belly-down. By the 1930s, the verb was used in reference to the much more efficient (but less frog-like) method of getting someone in an arm-behind-the-back hold and hustling him or her along. As a verb by 1884.
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frogman (n.)
"scuba diver in rubber suit," 1945, from frog (n.1) + man (n.).
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froggy (adj.)
1610s, "full of frogs," from frog (n.1) + -y (2). Meaning "frog-like" is from 1837. Related: Frogginess.
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froggy (n.)
1822 as a familiar name for a frog, from frog (n.1) + -y (3). As a disparaging term for a Frenchman by 1857.
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bullfrog (n.)
also bull-frog, large North American species of frog, 1738, from bull (n.1) + frog (n.1). So called for its loud voice.
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Rana 

frog genus, Modern Latin, from Latin rana "frog," which probably is imitative of croaking (compare frog (n.1)).

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rennet (n.2)

variety of apple, mid-15c., renette, from Old French rainette, diminutive of raine, reine "frog," from Latin rana "frog," which probably is imitative of croaking (compare frog (n.1)). If so, so called for its speckled skin like a frog's. The other possibility is that the Old French name of the apple is literally "little queen," a diminutive of reine "queen," from Latin regina (see Regina). OED, which leans toward this suggestion, suggests the other is folk etymology.

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