That word is informal even in American English. It's less common in British English, but it's growing in popularity in both versions, and I wouldn't call it "not a word" even in British English. But I disagree with its inclusion in a first lesson, because its main use over the more common standard alternative (rot13: "fbzrjurer") is signaling casual speech.
Google Ngram Viewer lets you compare popularity of words in British vs. American English, so it's useful for investigating this.
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That word is informal even in American English. It's less common in British English, but it's growing in popularity in both versions, and I wouldn't call it "not a word" even in British English. But I disagree with its inclusion in a first lesson, because its main use over the more common standard alternative (rot13: "fbzrjurer") is signaling casual speech.
Google Ngram Viewer lets you compare popularity of words in British vs. American English, so it's useful for investigating this.
https://books.google.com/ngrams