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).

*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."