Etymology
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polka-dot 

"pattern consisting of dots of uniform size and arrangement," especially on fabric, 1851 (polka-spot and polka-dotted are used in 1849), when they were in fashion, from polka (n.) + dot (n.). Named for the dance, for no reason except its popularity, which led to many contemporary products and fashions taking the name (polka hat, polka-jacket, etc.). They had a revival in fashion c. 1873. Related: Polka-dots.

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head over heels (adv.)
1726, "a curious perversion" [Weekley] of Middle English heels over head (late 14c.) "somersault fashion," hence "recklessly." Head (n.) and heels long have been paired in alliterative phrases in English, and the whole image also was in classical Latin (per caput pedesque ire).
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a la mode (adv.)
also alamode, 1640s, from French à la mode (15c.), literally "in the (prevailing) fashion" (see a la + mode (n.2)). In 17c., sometimes nativized as all-a-mode. Cookery sense in reference to a dessert served with ice cream is 1903, American English; earlier it was used of a kind of beef stew or soup (1753).
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a la 
from French à la, literally "to the," hence "in the manner of, according to," from à, from Latin ad "to" (see ad-) + la, fem. of definite article le "the," from Latin ille (fem. illa; see le). Attested in English in French terms from fashion or cookery since late 16c.; since c. 1800 used in native formations with English words or names.
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hay fever (n.)

also hay-fever, 1825, from hay + fever. Also called summer catarrh (1828); not much noted before the 1820s, when it was sometimes derided as a "fashion" in disease.

People are apt to sneeze, in hot weather for example; and people do not die of sneezing now-a-days, as they did in days that no one knows any thing about. We cannot give six draughts a-day, at one and nine pence each, for sneezing: call it the hay-fever. What a wonderful man! what a clever man! he understands the hay-fever: call him in! Thus is the hay-fever among the last in the list of fashionables. ["On Fashions in Physic," London Magazine, October 1825]
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Three Rs (n.)

1824; said to have been given as a toast by Sir William Curtis (1752-1829), a beloved lord mayor of London in the 1820s, who seems to have been a figure of fun to whom many mangled phrases were attributed. Among the toasts he is alleged to have given at public dinners were "The Female Ladies of London;" "The three C's—Cox, King, and Curtis;" and "The three R's—Reading, Writing, and Rithmetic."

It has been very much the fashion amongst a class of persons to attribute to Sir W. C. ... a vulgarity and ignorance of speech which are by no means consistent with his character and conduct. The worthy and hospitable baronet has a rapid mode of speech, but it is always correct ; and although some eccentricities are mixed up in his composition, he is highly honourable, and has been a very useful member of society, particularly to his London constituents. [The Mirror, Jan. 29, 1825]

After listing some examples, the article continues:

It is, however, very certain, that at a city festival some years ago, having indulged very freely, he fell asleep, when some wag, choosing to consider him dead, wrote his epitaph, which was found next morning pinned to the baronet's dress coat:—
"Here lies the great Curtis,
Of London, Lord May'r:
He's left this here world,
And gone to that there."
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