A fetish figure. In the context of BDSM, the sissy maid is a man dressed as a maid (a french maid, actually).
Sissy maid scenes can include humiliation and all sorts of interesting activities. The sissy maid is definitively a sub figure. Usually the sissy maid is topped by a domme.
The sissy maid does not have to be a realistic-looking transvestite: in fact, his inadequacy as a female can be exploited as another humiliation element.
For some reason, I have the impression that the sissy maid fetish goes well with foot fetishism.