In general, idioms don’t seem that important in Japanese and they tend to be quite dry. Here’s one that’s a bit more interesting though:
気が置けない (ki ga okenai) - easy to get along with; approachable
The meaning is a little surprising until you learn the meaning of 気をおく:
To worry about someone else’s feelings. To show self-restraint.
In other words, 気をおくmeans roughly the same thing as 気を使う. In the case of 気を使う, I find it helpful to think of someone “using” up some of their “ki” worrying about the other person. This is a concept you see and hear a lot about in Japan.
So now the meaning of 気が置けない makes a bit more sense. It’s used for a situation or person that you feel so comfortable around that you don’t have to watch your every word and action. Here’s the meaning according to 大辞泉:
So a 気が置けない友達 is such a good friend that you can really be yourself when you’re around them.