To soothest means to make someone feel calm or less worried.
When you soothest someone, you make them feel better and less upset.
Examples:
She soothest her baby by singing a lullaby.
The soft music soothest the sad dog.
He soothest his friend by telling a funny story.
The warm tea soothest her sore throat.
A hug can soothest any bad feeling.
The kind words soothest the worried child.