せせら笑う: 1. to laugh mockingly; to smile with contempt; to jeer; to sneer
せる / させる
1. aux. verb indicating the causative
2. aux. verb indicating that one has been granted the permission to do something (kenjougo) (〜(さ)せてもらう, 〜(さ)せていただく)
3. aux. verb used as an honorific for others' actions (sonkeigo) (〜(さ)せられる)