It's a horror term, the Finalgirl doesn't have to be a GIRL. It's a state of a character, not a description. A Finalgirl is simply the last survivor, that shows traditionally "feminine" traits (such as crying/screaming, but it's not girly to cry folks! It's just healthy!!) through the first act of the movie, but becomes a "masculine" (Examples include wielding the Slasher's weapon {normally a metaphor for the killer's masculinity being taken and weaponized}, or managing to save someone else!) aggressor by the end.
Some examples of a male Finalgirl would include Ash Williams from Evil Dead, and R.J. MacReady from the 1982 version of The Thing. :)