Hembra is one of those Spanish words that an English speaker isn’t initially likely to make any connection to, but knowledge of a few of the quirks that Latin underwent on its way to becoming Spanish soon enough renders the word recognizable as a descendant of Latin femina ‘woman.’
First, three-syllable Latin words with a strong stress on the first syllable sometimes lost their middle vowel: Latin femina would have become *femna.
Second, given two nasal consonants in a row, Spanish would have replaced the second one with a non-nasal sonant that has the same point of articulation: *femna would have become *femra.
Next, to smooth the transition from m to r, proto-Spanish speakers would have introduced a consonant to ease that transition: *femra would have become *fembra.
Independent of those changes, in many Latin words that began with an f, that initial sound gradually weakened to/h/ and then ceased to be pronounced at all, even though an h was retained in the spelling. As a result, *fembra would have become modern Spanish hembra.
French underwent a rather different development of femina that ended up producing femme (pronounced /fam/ in French). We sometimes encounter that word in the phrases femme fatale and cherchez la femme, both of which have negative connotations.
In a borrowing with neutral connotations, Spanish and English went back to Latin for the adjective femenino/feminine.
© 2014 Steven Schwartzman