A day when you say you are sick and don't go to work or school, even if you are not really sick.
Taking a sickie is usually not a good thing to do. It can make people feel like you are not being honest or responsible.
Examples:
He took a sickie and didn't go to work.
She's not really sick, she's just taking a sickie.
Taking a sickie is not a good idea.
I think I'll take a sickie and stay home today.
He takes a sickie every time he doesn't want to go to work.
She took a sickie and went to the beach instead of work.