When someone smirs, they smile in a smug, self-satisfied, or silly way.
Smirking can be seen as rude or annoying, especially if someone is smirking at another person's misfortune.
Examples:
He smirs when he thinks he's right.
She always smirs when she gets her way.
The cat smirs when it knocks things off the table.
The little boy smirs after playing a trick on his sister.
The villain smirs after capturing the hero.
The naughty elf smirs as he steals the cookies.