Is it permissible to name a newborn male "Rabbani"?

There is no impediment to naming with this name, as it does not involve servitude to other than God, nor glorification of creation, nor anything from the matters of Jahiliyyah (pre-Islamic ignorance). Furthermore, describing someone with it is permissible, as Allah Almighty says: "Why do not the rabbis and the religious scholars forbid them from saying evil and eating what is unlawful? How evil is what they have been doing" (Al-Ma'idah: 63). If describing with it is permissible, then naming with it is also permissible.