slack off

slack off (not work hard, be lazy) — лениться, лентяйничать; отлынивать от работы; сачковать, филонить (сленг.)

Example 1: I caught Jake slacking off today. He was supposed to be finishing that report. Instead he was playing a video game.

Example 2: Okay, I gotta be blunt. It appears to me that you are slacking off. Um, I can’t help but notice those files are still sitting on the desk. To me, that’s slacking off. That’s not working.

slack off in one's studies

Origin
The idiom comes from sailing. If something is slack, it means it’s loose. On a ship, there are lots of ropes that need to be kept tight. If you’re not doing your job, slack builds up.

Related vocabuary:
[прогуливать]
[work-shy]