Friends, could you help me with this phrase?
 

Shinとオレたちは刎頚の友だ -
I dont get to understand this "funkei".
Friends of decapitation?
is it an idiomatic expression? maybe old friends?
Thank you very much and sorry for annoy you again.

Did someone actually say this to you? If yes, your friend is the King of Japanese vocabulary. I've only heard/seen it a few times in my rather long life.

刎頚の友　is a literally expression meaning "the best of best friends". It's an "If you die, I'll die" kinda relationship.

Nagoyankee, thank you very much.
As ever you saved me here.
I try to read mangas in japanese and almost ever you from this forum sort my doubts out.
Thank you very much.
Maybe be anything about Harakiri ritual, right?

Right, you will find the term most often in historical novels. No living men are really old enough to say it in real life, except in exaggerated and/or dramatic speech. The term has a connotation that you wouldn't regret if that special friend of yours had to kill you. The two are as one.

I propose we all use this phrase once in our next Japanese conversation. It's like 鬮, the most epic kanji ever. I learned it a long time ago just to make humor about old Japanese with my friends at university in Japan. They'd never seen it, so it was a laugh to see a non-native scribbling away at this who-knows-how-to-count-that-high-stroke-count character.

Go ahead. Zoom in on that character and see how crazy it really is! ;)

And, to clarify, that character is pronounced くじ. As in たからくじ (lottery).

And, to clarify once more, it's archaic, and no one uses it anymore.

鬮

This is what it looks like...wow...nutty.