let one's hair down(The most common complaint of a West Ender would be about residents of the surrounding areas that arrive to let their hair down and act as they never would in their own neighbourhood, that is drink, fight, urinate under tables in cafes and harrass locals. ART Vancouver)
sing one's own praises("I am sure that you will not think that I am unduly singing my own praises if I say that my student career was considered by my professors to be a very promising one." (Sir Arthur Conan Doyle)ART Vancouver)
it takes one to know one(used for saying that someone must have a bad quality themselves if they can recognize it in other people
: ‘He's a complete idiot!' ‘Well, it takes one to know one!'Leonid Dzhepko)