late 14c., "the young while in the womb or egg," from Latin fetus (often, incorrectly, foetus) "the bearing, bringing forth, or hatching of young," from Latin base *fe- "to generate, bear," also "to suck, suckle" (see fecund).
In Latin, fetus sometimes was transferred figuratively to the newborn creature itself, or used in a sense of "offspring, brood" (cf. Horace's "Germania quos horrida parturit Fetus"), but this was not the basic meaning. Also used of plants, in the sense of "fruit, produce, shoot." The spelling foetus is sometimes attempted as a learned Latinism, but it is not historic.