An acronym for Female-to-Male Significant Other. The spouse/life partner of female to male transsexual. Most seem to come out of the lesbian community but many are self-identified straight women who don't meet their transman until after he's gone through some form of transition. Some transmen are married or partnered with men, too.
The process of transsexual transition can be a very disruptive process for partners. It calls into question many basic assumptions, especially sexual identity or sexual orientation.
Because our society doesn't have any postive public role models (sitcoms don't qualify), these people can feel very isolated. Though it is no tougher an issue than any relationship issue is (as a reason for divorce or separation), it is made harder by the fact that there are no social road maps or patterns to follow.
See also, SOFFA.