Etymology
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fetus (n.)

late 14c., "the young while in the womb or egg" (tending to mean vaguely the embryo in the later stage of development), from Latin fetus (often, incorrectly, foetus) "the bearing or hatching of young, a bringing forth, pregnancy, childbearing, offspring," from suffixed form of PIE root *dhe(i)- "to suck."

In Latin, fetus sometimes was transferred figuratively to the newborn creature itself, or used in a sense of "offspring, brood" (as in Horace's Germania quos horrida parturit Fetus), but this was not the usual meaning. It also was used of plants, in the sense of "fruit, produce, shoot," and figuratively as "growth, production." The spelling foetus is sometimes attempted as a learned Latinism, but it is unetymological (see oe).

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feticide (n.)
also foeticide, 1842; see fetus + -cide. Related: Feticidal.
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effete (adj.)
1620s, "functionless as a result of age or exhaustion," from Latin effetus (usually in fem. effeta) "exhausted, unproductive, worn out (with bearing offspring), past bearing," literally "that has given birth," from a lost verb, *efferi, from assimilated form of ex "out" (see ex-) + fetus "childbearing, offspring" (see fetus). Figurative use is earliest in English; literal use is rare. Sense of "intellectually or morally exhausted" (1790) led to that of "decadent, effeminate" (by 1850s).
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*dhe(i)- 

*dhē(i)-, Proto-Indo-European root meaning "to suck."

It forms all or part of: affiliate; affiliation; effeminate; effete; epithelium; fawn (n.) "young deer;" fecund; fellatio; Felicia; felicitate; felicity; Felix; female; feminine; femme; fennel; fenugreek; fetal; feticide; fetus; filial; filiation; filicide; filioque; fitz; infelicity.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit dhayati "sucks," dhayah "nourishing;" Greek thēlē "mother's breast, nipple," thēlys "female, fruitful;" Latin felare "to suck," femina "woman" ("she who suckles"), felix "happy, auspicious, fruitful," fetus "offspring, pregnancy;" fecundus "fruitful, fertile, productive; rich, abundant;" Old Church Slavonic dojiti "to suckle," dojilica "nurse," deti "child;" Lithuanian dėlė "leech;" Old Prussian dadan "milk;" Gothic daddjan "to suckle;" Old Swedish dia "suckle;" Old High German tila "female breast;" Old Irish denaim "I suck," dinu "lamb."

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oe 

a digraph written also as a ligature (œ) found in Latin words and Greek borrowings into Latin, representing Greek -oi-. Words with -oe- that came early into English from Old French or Medieval Latin usually already had been leveled to -e- (economic, penal, cemetery), but later borrowings directly from Latin or Greek tended to retain it at first (oestrus, diarrhoea, amoeba) as did proper names (Oedipus, Phoebe, Phoenix) and purely technical terms. British English tends to be more conservative with it than American, which has done away with it in all but a few instances.

It also occurred in some native Latin words (foedus "treaty, league," foetere "to stink," hence occasionally in English foetid, foederal, which last was the form in the original publications of the "Federalist" papers). In these it represents an ancient -oi- in Old Latin (for example Old Latin oino, Classical Latin unus), which apparently passed through an -oe- form before being leveled out but was preserved into Classical Latin in certain words, especially those belonging to the realms of law (such as foedus) and religion. These language demesnes, along with the vocabulary of sailors, are the most conservative branches of any language in any time, through a need for precision and immediate comprehension, demonstration of learning, or superstitious dread. But in foetus it was an unetymological spelling in Latin that was picked up in English and formed the predominant spelling of fetus into the early 20c.

The digraph in English also can represent a modified vowel, a mutation or umlaut of -o- in German or Scandinavian words (such as Goethe) and a similar vowel in French words (e.g. oeil "eye," from Latin oculus).

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chorion (n.)

"outer membrane of the fetus," 1540s, medical Latin, from Greek khorion "membrane enclosing the fetus, afterbirth," from PIE root *ghere- "gut, entrail." Related: Chorionic.

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embryo (n.)
"fetus in utero at an early stage of development," mid-14c., from Medieval Latin embryo, properly embryon, from Greek embryon "a young one," in Homer, "young animal," later, "fruit of the womb," literally "that which grows," from assimilated form of en "in" (from PIE root *en "in") + bryein "to swell, be full."
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amnion (n.)
innermost membrane around the embryo of a higher vertebrate (reptiles, birds, mammals), 1660s, Modern Latin, from Greek amnion "membrane around a fetus," originally "vase in which the blood of a sacrifice was caught," which is of unknown origin; sometimes said to be from ame "bucket," or a diminutive of amnos "lamb."
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