How can I find the etymology of an English word?

Most English words have known etymologies (word histories) that linguists have established and refined over time. Tracing a word back to its earliest recoverable form often involves uncovering several chronological layers. For the immediate ancestry of an English word, however, your first stop should be the Oxford English Dictionary (OED). The recorded ancestors of an English word can usually be found within the entry for that word in the OED online [Harvard Key required] or in the print version of the OED (2nd ed., 1989).

For example, the OED entry for Modern English coast shows that this word goes back to Middle English cost (spelled several different ways), which is a borrowing from Old French coste, which in turn descends from Latin costa 'rib, flank, side.' For this word the OED gives no further etymological background.

Etymologies may run much deeper, however. For example, the OED entry for Modern English yoke 'apparatus for animals used to pull a wagon or plow' shows that this word goes back to Middle English yok (spelled several different ways), which in turn goes back to Old English geoc (again spelled several different ways), which is first recorded over a thousand years ago during the Old English (OE) period (ca. 450-1066 AD).

But the ancestry of yoke goes back even further. As the OED shows, OE geoc has cognates (related forms) in older Germanic languages that are closely related to Old English, such as Old High German (joch), Old Norse (ok) and Gothic (juk).  Together these forms point to a reconstructible common ancestor or 'protoform' *juka-, which we may attribute to a prehistoric parent language that linguists call Proto-Germanic (spoken ca. 500BC).

The recoverable history of yoke runs even deeper than Proto-Germanic, however. As the OED shows, yoke has cognates elsewhere in the Indo-European language family, such as Latin jugum, Sanskrit yugá-m, and Welsh iau, among others.  Together these forms point to a reconstructible common ancestor *yugó-m, which we may attribute to the prehistoric parent language of the Indo-European family, which linguists call Proto-Indo-European (spoken ca. 3500 BC).

The OED is especially useful for finding older forms of Modern English words that are recorded in Old and Middle English texts. The OED is also generally reliable in its listing of a word's cognates in Germanic and elsewhere in Indo-European. For accurate reconstructions, however, further sources should be consulted (see below).

A concise companion to the OED is C.T. Onions, ed., The Oxford Dictionary of English Etymology (Oxford, 1966).  Reconstructions of Proto-Germanic forms found here are generally reliable.

For the most accurate and up-to-date reconstructions, however, users should consult the index to Calvert Watkins, ed., The American Heritage Dictionary of Indo-European Roots (3rd ed., Houghton Mifflin Harcourt, 2011)  The same material is included as an appendix to the fifth edition of The American Heritage Dictionary of the English Language (2011).

For deeper background on English words that descend from Latin, see Michiel de Vaan, Etymological dictionary of Latin and the other Italic languages (Brill, 2008).

For deeper background on English words of Greek origin, see R.S.P. Beekes, Etymological dictionary of Greek (Brill, 2010).