There is nothing obscure about the phrase "hold no brief". It may have begun as a legal term but is now and has been in general use for all of my lifetime at least (58 years).
The original poster said nothing about a 'beef', and what on earth has a google exact phrase result got to do with it ?. Any well read NS or listener to radio & TV will have heard the phrase a thousand times in it's modern usage. Perhaps you should broaden your scope a little.

As for 'Longdong', it seems you are not aware of the other connotation that the word has, maybe your head has been in the sand too long.