Here are some common idioms using "chicken":
* "Scaredy-cat" or "chicken": This is the most common and direct way to call someone cowardly.
* "Don't be a chicken": This means to be brave and not be afraid.
* "He chickened out": This means someone backed out of something due to fear.
* "You're a chicken if you don't try it": This is a challenge to someone to be brave and face their fears.
* "He's too chicken to tell her the truth": This means someone is too afraid to be honest.
It's important to note that using "chicken" to describe someone can be considered offensive. It's better to use more neutral language when discussing someone's fear or lack of courage.