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