godan verb (-su)
leave open; keep open
Used for doors, windows, or other openings that are left open, often carelessly or intentionally. The っ adds a colloquial, emphatic feel compared to 開け放す.
誰かがドアを開けっ放したまま出かけた。
Someone went out leaving the door open.
窓を開けっ放して寝
I caught a cold because I slept with the window left open.