1. Yellow-bellied
A yellow-bellied person is a coward.
2. (to have a) Yen
If you have a yen to do something, you have a desire to do it.
3. Yeoman's service
(UK) To do yeoman's service is to serve in an exemplary manner.
Examples:
1. Yellow-bellied
Art: Bill ran away from the enemy yesterday.
Vic: We cannot win the war if our soldiers are yellow bellied.
2. (to have a) Yen
Wilhelmina: I am going to the supermarket now.
Lydia: May I come with you? I have a real yen for strawberries.
3. Yeoman's service
Maxie: Willie did a great job on the project.
Maisie:Yes, he really did yeoman's service.