Via Twitter, theatre director Jen Bender posed a question that had recently come up in conversation: "A married man's lover is his mistress. What's the name for a woman's illicit lover?" Searching for an answer to that question points to the many gender-related asymmetries in English.

Earlier this year, Orin Hargraves devoted a Language Lounge column to just this type of asymmetry in our lexicon. Some words inevitably refer to women and have no exact male equivalent, such as coquette, jezebel, vamp, concubine, and spinster. As Orin notes, these words tend to cast women in a negative light, and they persist in the language despite the progress of the feminist movement.

So what about mistress, in the sense of "illicit female lover"? It would seem to fall into the same category of words for women with questionable lifestyles, while men who assume the same role don't get their own gender-specific label. Mistress in its early usage was paired with master, of which it is the feminine form. But when the adulterous meaning got attached to mistress, master did not follow suit. The Oxford English Dictionary shows the relevant sense becoming popular in the 17th century, as in this quotation from John Donne: "Those women, whom the Kings were to take for their Wives, and not for Mistresses, (which is but a later name for Concubines)."

The online edition of the OED now includes a historical thesaurus (a truly great resource), which allows you to click on the sense of a word to find synonyms and near-synonyms throughout the history of English, arranged chronologically. Selecting the "illicit female lover" meaning of mistress reveals a panoply of words specific to women, 45 in all — from chevese in Old English, to doxy, pinnace, nobsy, and underput in early Modern English, to 19th-century terms like fancy-woman, fancy-girl, fancy-piece, poplolly, leveret, and other woman.

But if you click over to the "illicit male lover" category in the semantic hierarchy of the OED's historical thesaurus, the pickings are slim. From Middle English, there is leman and concubine, but these words were more often applied to women. Not until the 19th-century are there terms that are specifically male: fancy man and other man, which are clearly modeled on fancy woman and other woman.

While we're lacking a good mistress equivalent, there are words that can apply equally to "other" men and women, namely lover and paramour. When I checked out the long-term patterns of usage of these words on the Google Books Ngram Viewer, I noticed something interesting. While the two words are ostensibly gender-neutral, they have historically been used much more often for male referents. Compare how her paramour has outpaced his paramour, and even more starkly, how her lover has outpaced his lover. (I am assuming, of course, that historical texts in these cases would by and large refer to heterosexual relationships.)

It seems that the availability of mistress and various other female-only alternatives has allowed these ungendered terms to swing toward men. There are, no doubt, other explanations for these patterns: in the case of lover, which has a broader semantic range than paramour, perhaps "loving" (of various kinds) has in the past been thought of as an activity that men engage in more than women. (Jen Bender says the idea makes her think of "a young troubador wandering the streets, professing his love.") Whatever the reason, this set of terms for clandestine extramarital partners is further proof that our language has deep, structural imbalances in describing the sexes.

(Update: The delightful Twitter persona GRAMMARHULK suggests another male equivalent for mistress, borrowed from Italian: cicisbeo. I've seen that term suggested elsewhere online, but I'm not sure it's such a good fit. In Italy in the 18th and 19th centuries, a cicisbeo was the "professed gallant" who attended a married woman, also known as a cavalier servente. It's really more of a male equivalent for courtesan. For more, see James Harbeck's "Word Tasting Note" on his Sesquiotica blog.)