From Wiktionary, the free dictionary
- drinne (more usual in Northern/Western German speech)
- drin (also often in this sense)
From dr- + innen; the first component is from a short form of dar-.
drinnen
- inside (a room)
- Antonym: draußen
- “drinnen” in Duden online
- “drinnen” in Digitales Wörterbuch der deutschen Sprache