What does "sleep in" mean?

To sleep late; to go on sleeping past one's customary or planned hour.

What is a synonym for "sleep in"?

Common synonyms include: live in, oversleep, oversleeping, oversleeps, overslept, sleep late.

/ˈsliːp ˌɪn/ verb

verb ·Rare ·Advanced level

Verb

1
To sleep late; to go on sleeping past one's customary or planned hour. idiomatic, intransitive
"On a rainy Saturday, after a busy week at work, he closed the curtains and decided to sleep in."
2
sleep later than usual or customary wordnet
3
Used other than figuratively or idiomatically: see sleep, in. intransitive
"She didn't come home last night — her bed hasn't been slept in."
4
live in the house where one works wordnet

"On a rainy Saturday, after a busy week at work, he closed the curtains and decided to sleep in."