A “Slacker” is a person who is really lazy and does less work than they should do. People can be slackers in school or at work. If you do this sometimes, it’s called “slacking off“.
Here are a few example sentences:
He is a smart guy but he’s such a slacker. I think the boss is going to fire him if he doesn’t improve.
She slacked off the whole semester but studied really hard for the exam. She somehow got the top mark in the class.
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