Laudator Temporis Acti

Anatoly Liberman, the Oxford Etymologist, On Hobos, Hautboys, and Other Beaus. Also, On Suffixes as Midwives:

Unless we can explain why the phenomenon under investigation arose where and when it did, we have no right to speak up. Since etymology habitually attracts the attention of lay people, they often believe that any ingenious suggestion has the right to exist or that one can guess the origin of a word by developing a feel for it. This is a dangerous illusion. Tracing words to their sources is a scholarly pursuit.

Liberman gives examples of the unscientific methods likely to be followed by unscholarly amateurs:

Let us suppose that we are trying to find out the origin of the word devil. Here are a few excellent possibilities. 1) There once was a vicious man called Deville. The crimes he committed were so terrible that his name became synonymous with Satan. 2) The devil is evil, and this is how the word arose (letters come and go, so never mind initial d: compare Ned from Ed and whilst from while; a school teacher I knew—his name was Ned—used to send his students to the room acrosst the hall). 3) The devil makes people deviate from the straight and narrow path of virtue. Devil is, obviously, dev- (from deviate) plus ill. 4) The devil is resilient but ugly. His color is livid, for he has been beaten and bruised innumerable times. Devil is livid pronounced from right to left and slightly altered (people have many reasons to avoid the use of the word devil: compare Old Nick and other euphemisms like it).

Morgan Kavanagh, Origin of Language and Myths (London: Low, 1871), actually adopted one of the methods described by Liberman — reading a word backwards in a misguided attempt to discover its origin. Ernest Weekley, Something About Words (London: John Murray, 1935), pp. 167-168, gives specimens of Kavanagh's reasoning:

Among our author's minor discoveries are the convenient fact that 'any letter can become any other letter', and the equally helpful law that all words may be read backwards as well as forwards, 'for why should English not have the same privileges as Hebrew?' The advantage of the Hebrew method may be illustrated by the following example—'If we read spot, a place, from right to left, what shall we obtain if not tops, and what is tops, when the vowel due between the p and s is supplied, but topos, and this is the Greek for place. When in like manner we read skin from right to left, what have we? Niks; and as here the i has o understood, and as o and i make a, we obtain naks, of which nak is the radical part of naked, and to be in one's skin is to be naked.'

William Bryant Logan, in his books Dirt: The Ecstatic Skin of the Earth (New York: Riverhead Books, 1995) and Oak: The Frame of Civilization (New York: W.W. Norton & Company, 2005), often discusses the origins of words, usually (but not always) accurately.

On p. 38 of Dirt, Logan writes, "It takes dirt to grow an oak from an acorn. It takes the rot and the shit that is the root meaning of 'dirt'—dritten means 'shit' in Old Norse."

It's unclear whether Logan thinks dritten is a noun or a verb in Old Norse. The noun is drit, and the infinitive of the verb is apparently dríta: see L.F.A. Wimmer, Altnordische Grammatik, tr. E. Sievers (Halle: Waisenhaus, 1871), p. 104.

I'm way out of my league here, though, since I'm an amateur in the field of etymology, and I know nothing about Old Norse. But according to the experts, there is definitely a connection between the Old Norse word and the English word. Oxford English Dictionary (OED) gives the following etymology of dirt:

By metathesis from ME. drit, not known in OE. and prob. a. ON. drit neuter, excrement (mod. Icel. dritr masc., Norw. dritt); cf. also MDu. drete, Du. dreet, Fl. drits, drets excrement: see DRITE v.

Metathesis is the "transposition within a word of letters, sounds, or syllables, as in the change from Old English brid to modern English bird." (American Heritage Dictionary).

The first meaning of dirt in the OED is "ordure" (excrement) and the first example of the meaning "soil" is late (1698). So dirt itself, one could say, is a dirty four-letter word.

Logan's etymological speculation on p. 80 of Dirt is more doubtful:

The digging sticks of primitive tribes were often made explicitly in the shapes of phalloi, just as are the dibbles sold in every garden catalogue today. (Indeed, I have often wondered whether "dibble" is the diminutive of "dildoe.")

This is highly unlikely, if only on chronological grounds. Dildo is first attested at the end of the 16th century in English, dibble about 150 years earlier. It is unlikely that a diminutive would surface so long before the word it diminishes. I suspect that the etymology is also unlikely on phonetic grounds.

OED calls dildo "A word of obscure origin." One of the OED quotations, from Richard Burton's Arabian Nights X.239 (1886), connects it with Italian diletto (= delight):

Of the penis succedaneus,..which the Latins called phallus and fascinum, the French godemiché and the Italians passatempo and diletto (whence our 'dildo'), every kind abounds.