When someone shows sassiness, they are being a little disrespectful or cheeky in a playful way.
Sassiness can be seen as confident and fun, but it can also be seen as rude or impolite, depending on the situation.
Examples:
Her sassiness always makes us laugh.
I don't like your sassiness, young lady!
He showed his sassiness by sticking out his tongue.
The cat's sassiness made everyone smile.
Her sassiness was off the charts today!
The little boy's sassiness was cute and funny.