From A Hard Day’s Night:
What does this all mean in standard American English?
Not having seen the film in many years I’m not sure of the context, but I think it refers to ‘Breach of Promise’, a now-obsolete matrimonial cause of litigation, which dates from the time when betrothal is considered almost as binding as marriage itself.
I recently started a thread in the Pit about a recent case. In some parts of America breach of promise is still an offence.
Marrying someone under false pretenses (I’m an American though). Great movie BTW.
Not quite. It means promising to marry someone, and then not marrying them. The theory behind this was that a woman who expected to marry a man might indulge him in certain favors and make certain plans that she would not have if she were not about to be married to him, thereby incurring certain damages.
It means basically: he is a villain, a troublemaker and a cad.
Huh. I was born in England in 1953, lived here ever since and only the phrase “He’s a villain” means anything to me. 
The OED gives “mixer,” meaning a troublemaker as UK slang dating from 1938. The next cites are from A Hard Day’s Night. It’s possible that this was a common bit of slang in the Liverpool area but not throughout the country.
And a mountebank.
I’m from the South East and London, I seem to remember someone who used to work with who was born in the 1940s and had a solid Berkshire working-class background use it. I certainly knew what it meant instantly.
But he’s very clean.
And certainly uncontaminated by cheese.
+1 to each of you!
it was also seen as damaging to the woman’s reputation if people were to conclude that the engagement was broken due to her immorality/unfaithfulness.
It’s amazing how a little throwaway comment leads to the fighting of more ignorance. I didnt’ know about anything called “The Mountebank”.
There’s an old song I heard from Ian Whitcomb, from the perspective of a Jewish boy who’s found a gentile girlfriend. One portion is:
Mother, Mother, Mother
Your boy is a mixer
Take down the mezuzah
I’m bringing home a shiksa!
In this example there’s a double meaning of MIXER as “troublemaker” and that the specific trouble he’s going to cause is by not limiting his romantic entanglements to his own kind.
This site gives the quote thus:
So, yeah, it’s breach of promise of marriage, the civil action that a woman could take against a man who breaks off a valid engagement without reasonable cause. The action wasn’t abolished in England until 1970
As others have said, it’s not “future promise cases” but rather “breach of promise cases.”
Warning: the explanation requires the recounting of lots of sexist and misogynistic ideas about women and chastity.
Basically the guy has a reputation for bedding women and then leaving them. Said women’s families’ then bring a legal action against the cad for robbing their innocent girl of her virginity under the pretense that they would be married.
Under the common law, a man could face penalties for leading a virtuous girl to believe that he would marry her and then breaking that promise. The girl and her family were considered to have suffered harm, because that girl would find it difficult to then find a suitable husband, being under the suspicion of not being a virgin.
That was called a “breach of promise” action.
Further up-market (which was where most breach of promise cases tended to arise), financial contracts and political/territorial/business alliances might also have been involved. In the days when upper crust marriages were almost akin to alliances between nation states, backing out of a marriage contract might have all sorts of consequences beyond a woman’s reputation.
But hardly applicable in the OP’s example, I grant: that’s just describing a louse.
I’ve heard that explanation (not that I’ve devoted a lot of research to it) but my understanding was that it was an exception to the general rule of contracts that a mere promise did not create a binding contract. However, in the case of marriage, public policy wanted to ensure that a man took a promise to marry with due solemnity and did not propose marriage lightly or subject a woman to the expectation of being cared for by him and then he breaks the promise for no good cause.
And back then, the “good virtuous” woman would wait until the wedding night for sexual intercourse anyways (at least in theory). I cannot imagine that the law would countenance that it needed a tort for fornicating women because of a promise to marry.
Wouldn’t that fall under the in pari delicto doctrine? Nobody can profit from their own criminal activity. If a woman’s reputation was sullied because she broke the law by committing fornication, I would argue (if I was a barrister in 1587) that the woman cannot sue for harms arising from her own criminal actions. IOW, if she couldn’t find a husband because she committed fornication, well, that’s her fault.*
*Again, with all of the disclaimers that you made about how this is terribly anachronistic, but so is the tort itself.