Extract from W. W. Skeat's Etymological Dictionary of the English Language (2nd edition - 1883) - chapter xxiii.]
In the course of the work, we have been led to adopt the following canons which merely express well-known principles and are nothing new. Still, in the form of definite statements, they are worth giving.
1. Before attempting an etymology, ascertain the earliest form and use of the word and observe chronology.
2. Observe history and geography. Borrowings are due to actual contact.
3. Observe phonetic laws, especially those which regulate the mutual relation of consonants in the various Aryan languages, at the same time comparing the vowel-sounds.
4. In comparing two words A and B belonging to the same language of which A contains the lesser number of syllables, A must be taken to be the more original word unless we have evidence of contraction or other corruption.
5. In comparing two words, A and B, belonging to the same language and consisting of the same number of syllables, the older form can usually be distinguished by observing the sound of the principal vowel.
6. Strong verbs in the Teutonic languages and the so-called Irregular Verbs in Latin are commonly to be considered as primary, other related forms being taken from them.
7. The whole of a word and not a portion only ought to be reasonably accounted for and in tracing changes of form any infringement of phonetic laws is to be regarded with suspicion.
8. Mere resemblances of form and apparent connection in sense between languages which have different phonetic laws or no necessary connection are commonly a delusion and are not to be regarded.
9. When words in two different languages are more nearly alike than the ordinary phonetic laws would allow, there is a strong probability that one language has borrowed the word from the other. Truly cognate words ought not to be too much alike.
10. It is useless to offer an explanation of an English word which will not also explain all the cognate forms.