It turns out that etymology is my crack. I can't resist it. Yesterday, after posting the AN question about freemartins, I couldn't resist looking just a bit further for the origins of the word, and found this from a 75-year old Journal of Agricultural Research*:
The freemartin has been known to cattle breeders since before the establishment of the Roman Empire. The sterile cow born twin with a bull was referred to by Varro, a writer who died in 28 B. C.
It was called "taura," which apparently meant "barren cow." Although the condition has been recognized for some 2,000 years the origin of the term "freemartin" is obscure. According to one authority the word "free" meant "willing" or "ready to go," as the freemartin was supposed to be an especially willing worker. It has been proposed also that the word "free" was used to signify exemption from reproduction (sterile). Another authority saw in the term a contraction of the words "ferry," "ferow," or "farrow," which appear to be associated with the Flemish "varvekoe"—a cow that gives no milk—and with the West Flemish "varwekoe"—a cow that has ceased to be capable of producing offspring. It is not difficult to imagine an association between the two words "free" and "farrow."
There is probably greater speculation about the word "martin." It may have been derived from the Irish and Gaelic "mart" meaning heifer or cow. Efforts have been made to trace it to St. Martin who, according to legend, once cast the devil from a cow. Moreover, St. Martin is said to have been the patron saint of twins and unusual fecundity. Another explanation offered is that on or near November 11, which was called Martinmas day in Scotland and England, it was customary to slaughter cattle the meat of which was salted for winter use and called martinmas-beef. An early English dictionary referred to martin as "not a true heifer, but an undeveloped male with many of the characteristics of the ox, and generally fattened and killed about Martinmas." It has been suggested further that the freemartin may have been given that designation because its meat was so choice that it was reserved for St. Martin's—a great feast day. Moreover the words "mart," "maert," "mert," and "mairt" appear to have been used in Scotland and parts of England in referring to the cow or ox fattened for slaughter and salted or smoked for winter use.
Hart showed that it is not difficult in view of these facts, to imagine such an individual being referred to as the "farrow-mart-one," or in Scotland as the "farrow-mart-yin," either of which might have been corrupted or shortened into "freemartin."
This is why there's been a reasonable amount of observable behaviour: freemartins will mount a cow in œstrus but not hurt it or (of course) be able to impregnate it. So before blood tests and thermometers, farmers might have used freemartins to tell when their cows are coming into season. Knowing this makes all the more obvious the contortions the authors of paper go through to avoid mentioning sexual behaviour. I'm sure it must have occurred to them that another way to regard "free" is to approach it from the "free with her favours" angle (especially when linked to their phrase "ready to go"). Either they were coy on their own behalf or at the direction of their editor, or they were utterly clueless about sex. And if you've spent time on a farm, one thing you are not is clueless in this regard...
The stuff about St Martin is interesting, too. I'm assuming they mean Martin of Tours (though the fact that he's the patron saint of twins is new to me). He lived before Hild, but I'm not sure how well known he was in her time. Not very, I'm guessing. So if she used the word freemartin, it came from somewhere else. But that is definitely an investigation for another time. Maybe.