Today I came across a song that I haven't heard for ages called 'friend is a four letter word'. I used to listen to it in the past without really thinking about the lyrics but today I paid attention to the title and realized that there was something wrong with it. In fact when I first saw the sentence 'friend is a four letter word' it pretty confused me because the word friend doesn't have 'four' word. Therefore I thought that it was a kind of figure of speech and googled it. I found several translations so I still don't understand what it means.
Can you guys please tell me what it means and in which context would you use it?
It is an offensive way to expression: https://en.wikipedia.org/wiki/Four-letter_word
For example the word (my apologies) "fuck". It has 4 letters.
In your sentence, "friend is a ____ (offensive word with 4 letters)"