Reepicheep's weapon is a small rapier that is just the right size for him. He carries it nobly, using it to defend himself and Narnia in many battles and duels. When he is insulted and ill-used by Eustace, he uses the flat edge of his rapier to punish Eustace by whipping his hand smartly.
When Reepicheep leaves on the coracle to enter Aslan's Country, he expresses that he no longer has need of his sword, and throws it into the sea of lilies, where it sticks upright into the shallow water, and perhaps is still there today.