I found this on Internet
(Les scientifiques utilisent le terme de free-martin (ou freemartin) issu vraisemblablement du dialecte écossais ferry = stérile et mart = animal impropre à la reproduction, ou animal destiné à la boucherie et vendu à la foire de la Saint Martin. )
I try to translate for you
The scientists use the term freemartin. This term come from scottish slang. ferry = sterile and mart = animal no good for breeding.
The two word togetter (ferry mart) became freemartin
They say it could be animal no good for breeding sold to the market the day of St-Martin .
Maybe there is others signification depend of the country.