There are many ways to refer to one's mother-in-law without using the actual term. Some common alternatives include:
My husband's mother
My wife's mother
The woman my husband married (or The woman my wife married)
That lady over there (if pointing her out in a crowd)
Your/His/Her mother
Assuming you want to avoid saying mother-in-law because you don't have a good relationship with her, it might be best to simply refer to her by her first name. For example, I'm going to go talk to Jane. If that feels too awkward or informal, you can always use a title such as Mrs. Smith (if you know her last name).