1. A "nightmare" of a marriage - a marriage that only creates a living hell for one or both members.
2. A hastily planned marriage; one that practically happened overnight. Usually leads to definition 1 anyway.
1. Liz's husband turned into such a douche after they got married... now she's totally stuck in a nightmarriage.
2. Bill and Wendy got married before they even knew eachother. It's a total nightmarriage. I don't think it'll last.