Popular Etymology

The Oxford English Dictionary reserves its most withering scorn for the peddlers in popular etymology.

The Words

acorn

From oak: oak-corn, oak-horn. Or the Gr. ακρον top, point, peak
From Goth. akran, probably from Goth. akr field, originally open unenclosed country, the plain. Hence akran the fruit of unenclosed land, natural produce of the forest, extended to fruit generally, and gradually confined to the most important forest produce, the acorn.

admiral

From med.L. admirari to wonder at.
From the Arabic am r commander, as in emir. Often followed by -al- of the, as in am r-al-bahr commander of the seas. Christian writers wrongly took am r-al- to be a substantive.

antimony

According to Johnson, in his Dictionary: the stibium of the ancients, by the Greeks called στιμμι. The reason of its modern denomination is referred to Basil Valentine, a German monk; who, as the tradition relates, having thrown some of it to the hogs, observed, that, after it had purged them heartily, they immediately fattened; and therefore, he imagined, his fellow monks would be the better for a little dose. The experiment, however, succeeded so ill, that they all died of it; and the medecine was thenceforward called Fr. antimoine antimonk. According to Smart, in Walker Remodelled: probably so called because, being seldom found pure, but mostly mixed with other metals, it seems repugnant to solitude. Presumably from Gr. μονος alone.
Probably, like other terms of alchemy, a corruption of some Arabic word, refashioned so as to wear a Gr. or L. aspect. Perhaps from Arab. ithmid, whence Arab. uthmud, whence L. athimodium, atimonium, antimonium. The popular etymology is, as usual in such cases, supported by an idle tale; however the chemist Basil Valentine is from the end of the 15th century, and the word was already used by Constantinus Africanus of Salerno at the end of the 11th century.

artichoke

An artichoke has a choke or chock in its heart. Or, through some connection with It. arci- chief, ciocco stump, or F. -chou cabbage, or L. -cactus the cardoon.
From the Arab. al-kharshuf, via O.Sp. and north.It., in various latinized forms. The word choke in this sense derives from the popular etymology.

avocado

Associated with Sp. avocado lawyer.
According to the Online Etymology Dictionary, from Aztec ahuacatl testicle, so called for its shape. The prim OED only tells us the word ahuacatl, not the meaning!

belfry

From the bell it houses.
Most likely from OHG. berg- to protect, defend, and fridu place of shelter, safety, the whole meaning 'defensive place of shelter', an obvious description of a pent-house fitted to ward off missiles from those to whom it gave shelter during siege operations. Whence watch tower, alarm-bell tower, bell tower, place where bell is hung.

brake

An alteration of break, since we use brakes to break our motion.
Perhaps from OF. brac arm, as in braquer le timon to turn the rudder, hence meaning lever. Or related to mod.Du. braak a flax-break, a toothed instrument for braking flax, extended to refer to a nose-ring for a draught ox by confusion with breaking a horse.

carnival

From L. carne meat, and vale farewell; a festival immediately preceding Lent.
From med.L. carnelevarium, from levare putting away.

quiz

According to B.H.Smart, in his 1836 Walker Remodelled: quiz, something to puzzle; one whom an observer cannot make out, an odd fellow; also to examine narrowly with an air of mockery. All of these words, which occur only in vulgar or colloquial use, and which Webster traces to learned roots, originated in a joke: Daly, the manager of a Dublin play-house, wagered that a word of no meaning should be the common talk and puzzle of the city in twenty-four hours; in the course of that time the letters Q,u,i,z were chalked or pasted on all the walls of Dublin with an effect that won the wager.
Of obscure origin. Perhaps from in-quis-ition. Smart's anecdote is omitted in his 1840 edition.

Sources

The Oxford English Dictionary, 2nd edition

The finest dictionary of any language. Online at dictionary.oed.com. Rumour has it there is a version for mobile phones.

Samuel Johnson's Dictionary of the English Language

Some of the juiciest definitions are here.

B.H. Smart's Walker Remodelled (1836)

A New Critical Pronouncing Dictionary of the English Language: adapted to the Present State of Literature and Science, embodying the Original Stories of Johnson, the Additions of Todd and Webster, and Many Words in Modern Use not included in Former Dictionaries, exhibiting the Pronunciation of Words in Unison with More Accurate Schemes of Sounds than any yet Furnished, according to Principles carefully and laboriously Investigated: explaining their Meaning by Classification and Mutual Reference, as well as by Improved Definitions. Smart also gives us instruction on the Principles of Remedy for Defects of Utterance. The common standard dialect is that in which all marks of a particular place of birth and residence are lost, and nothing appears to indicate any other habits of intercourse than with the well-bred and well-informed, wherever they may be found... A person needs not blush because he cannot help betraying he is a Scotchman or an Irishman; but it may nevertheless be an object of ambition to prove that his circle of intercourse has extended much beyond his native place.

Douglas Harper's Online Etymology Dictionary