It's the 'Unarmed Combat' skill that grants the ability to use Unarmed Combat. There's a 'non-incrementable' and an 'incrementable' version, the latter improves the effectiveness of the former's techniques (and more advanced combat techniques from later ones like Brawling.) You'll get a combat menu, and can put 'punch' and 'jump' on your quickbar... but attacking rabbits isn't the best way to kill them, since they start running the moment you start fighting them and run very fast.
The best way to kill them is to sneak right up next to them, and then pick them up in your inventory (requires four empty slots in a square, or it'll just run). Then you can select 'Wring Neck' to get a butcherable corpse. It's not intuitive, but it works. The Hunting skill is what's required to do this.