What is the suffix of Britain?

The word "Britain" doesn't have a suffix. It's a complete word on its own.

However, you might be thinking about the suffixes used in words derived from "Britain," such as:

* -ish: Used to form adjectives meaning "of or relating to" a place or people, like "British."

* -er: Used to form nouns meaning "a person from" a place, like "Briton."

Let me know if you had a specific word in mind, and I can help further!