An FTM (female-to-male transgendered/transsexual person) or other genderqueer, who is sexually attracted to other men, whether they are male-born, transgendered, transsexual, or what. It is shorthand: boy plus boy plus transgendered identity, in one word.

Like "fag," this is a term best used by those who identify with it in some way, rather than by outsiders. Unlike "fag," "trannyfag" is generally obscure enough that it is not used as a slur by homophobes or transphobes.

It can be a difficult identity to explain. As one trannyfag on the Sphere list remarked when queried about his identity, "I have begun answering with 'I'm a fag, but whether you take me as such really depends on what gender you think I am.'" That begins to illustrate the complexity inherent in this identity. It's very common for people to confuse gender and sexuality - for example, to look at a lesbian couple and demand, "So, which one of you is the man?" A term like "trannyfag" or "transdyke" combines sexual identities with gender and sex changes, confusing people even more.

In 2002, Canada's Xtra! magazine ran an essay by a trannyfag who found himself dating a woman. His experience illustrates the fluidity of gender...

That first night, gender had been irrelevant. I was so unsure of the gender of my person of desire, that when I was unable to find my him or her, I searched both the men’s and women’s washrooms. No luck in either. Later, and I’m not sure how much later, she told me she was a woman and a dyke.
...as well as the way that they destabilize the world's ideas of what gay, straight, male, and female should look like:
Most of the time I am a bearded, breasted, trannyboy who is with a fabulous lesbian lover. In the eyes of the law we are two women, and in the eyes of the straight majority we are hetero-sexuals. At times we are fags. Separately, we get read as a dyke and a fag. Together, our opportunities for coming out and challenging expectations have shifted. We challenge the assumptions of as many queer heads as we do straight people, and challenge others to see how our identities as individuals have not changed in the context of our relationship.
There are as many ways of being a trannyfag as there are of being a "normal" gay man; this is only one person's experience. But it is a useful look at an identity that seems straightforward and holds much more inside.

Quotes are from http://www.xtra.ca/site/toronto2/arch/body1218.shtm. More information can be found on the trannyfags listserv at http://groups.queernet.org/group/trannyfags.