sleep-in noun 1/ˌsliːp ˈɪn/ verb 1forms 4 noun1 •An instance of sleeping in, sleeping late, sleeping until one feels fully refreshed rather than rising at a prespecified time.I didn’t have anything to do Saturday morning, so I decided to have a sleep-in. sleep in /ˌsliːp ˈɪn/ verb1 •To sleep late; to go on sleeping past one's customary or planned hour.On a rainy Saturday, after a busy week at work, he closed the curtains and decided to sleep in. formsslept in · sleeping in · sleeps in · sleep-ins