On another forum someone called paralian said

"I looked it up in the dictionary on my laptop:
mésalliance |ˌmāzəˈlīəns; ˌmāˌzalˈyä n s|
noun
a marriage with a person thought to be unsuitable or of a lower social position.
ORIGIN French, from més- 'wrong, misdirected' + alliance (see alliance )."

and someone called fuelair said this in a couple of posts:

"it is an old term which basically means you met and 'got with" someone who was not good for you. It's current equivalent is, indeed, misalliance (aka mis-alliance) [...] specifically, to marry beneath oneself. French, and I am 98.5% certain I have seen it used in the Cadfael mysteries series at least twice - as well as others for that very general historical period."