Leaving something out or leaving something turned on — an informal expression indicating that something is left as-is (not put away or switched off).
朝から窓を開けっぱなしで寝てしまい、寒くて目が覚めた。
I left the window open since morning and fell asleep; I woke up because it was cold.
お皿を洗わずに出しっぱなしにすると、虫が寄ってくるよ。
If you leave dishes out without washing them, insects will come.
電気をつけっぱなしにしないで、節電を心がけよう。
Don't leave the lights on; try to save electricity.