Sullying means making something dirty or making it less good. It can also mean damaging someone's reputation or making them look bad.
Sullying has a negative feeling. It's not nice to do. It can make people feel sad or angry.
Examples:
He is sullying the white shirt with mud.
Sullying someone's name is not kind.
She is sullying the clean floor with her dirty shoes.
The rain is sullying the nice sidewalk.
The bully is sullying the good boy's name.
Stop sullying my favorite toy with your dirty hands!