If we assign each letter a respective integer, starting from 1, then a is 1, b is 2, c is 3, and so on. After z, the letters loop back around, but with a in front (aa, ab, ac). It then goes to ba, bb, bc... After this is completed, as you may have figured, another letter is added (aaa, aab, aac). "Prime letters" would be letters that are associated with a prime number. b would be the first prime letter, followed by c, e, g, et cetera.
Given an input n, find the nth "prime letter."
This is code golf, so the shortest answer in bytes wins!