Unexpectedly, the words fawn, meaning a young deer in its first year, and fetus (or foetus), meaning an unborn or unhatched offspring of a mammal, are doublets: they go back to the same etymological source but differ in form and meaning. While fetus has remained identical to this source, the form fawn is the result of sound changes—cf. also turban – tulip, clock – cloak, pastiche – pastis and lobster – locust.
The noun fawn originally denoted the young of any animal. For example, The Myrrour of the Worlde (1481), a popular encyclopaedia published by William Caxton (circa 1422-1491), contains the following:
The lyonnesse hath the first yere five fawnes.
The English fawn is from an Old French word of the same general meaning, variously spelt feün, foün, feon, fan, etc., and faon (the Modern French form).
The spelling faon is due to the influence of two other animal names: taon, meaning horsefly, gadfly (from Latin tabone), and paon, peacock (from Latin pavone).
The French word is from an unattested Vulgar Latin feto/-onis, from Classical Latin fetus, also incorrectly fœtus, a spelling which has no etymological basis.
As a noun, this Latin fetus meant young, offspring, progeny, brood. It also meant a bringing forth, bearing, dropping, hatching of young.
As an adjective, fetus meant fertile, pregnant with, full of, having newly brought forth.
This Latin fetus is the past participle of an unattested archaic verb fe-, which meant to generate, produce, and is also the base of:
– fecundus, the origin of English fecund and of French fécond(e)
– femina, the origin of English feminine and female, and of French féminin(e) and femelle
– fenus, or fænus, which meant in particular gain, profit, advantage
– fenum, or fænum, which meant hay – hence French foin, meaning hay, fenaison, meaning haymaking (time) and the verb faner, meaning to ted (hay) and to wither, fade
– felix, which meant fruit-bearing, fruitful, fertile, productive, hence of good omen, favourable, fortunate, and lucky, happy. This is the origin of English felicity and felicitate, and of French félicité and féliciter.