The word "chicken" can be used to talk about someone who is afraid to do something.
There's nothing to be scared of. Don't be a chicken."
"Everyone thinks I'm a chicken because I didn't go bungee jumping with them."
"Stop being a chicken and just go."
A: "Did you ask Martha on a date?"
B: "No. I was about to, but I got a little nervous."
A: "You're such a chicken."
Other Common Sentences
"I didn't know you were afraid of insects. You're a scaredy cat."
"He's a coward. He wouldn't even go on the rollercoaster ride."
"Jack is scared of everything. He's such a sissy."