It’s hard to build a world from scratch. So when I see great examples, I take notes.
Book: Days of Blood and Starlight by Laini Taylor
World building skill: Common Expressions
Don’t sheathe your claws: a chimaera saying that means “don’t hold your breath,” but with an undertone of preparing for coming danger.
When Karou sarcastically remarks to Ten, the White Wolf's minion, that she expects a thank you from him for making him a prettier body, Laini Taylor could easily have let Ten reply "Don't hold your breath." But at this point in the story, it is not clear yet whether Ten and the White Wolf are people she doesn't like or whether they are enemies. By instead inventing "Don't sheathe your claws" for Ten to say to Karou, Taylor is able to add a menacing edge that foreshadows their coming conflict. And on a world-building level, this phrase is also an opportunity to remind us that we are in a world of beasts with their own language, which has sayings that reflect chimaera culture and history.