〜っぱなし
left on, left as is (carelessly); nonstop
Verb ます-stem + っぱなし
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っぱなし means something was started or done and then left that way, when it shouldn't have been: 電気をつけっぱなしで寝てしまった, "I fell asleep with the light left on". 水を出しっぱなしにしないで, "don't leave the water running".
It carries a note of carelessness or complaint, unlike たまま, which is neutral, and ておく, which is deliberate.
It also describes doing something nonstop, usually tiringly: 一日中立ちっぱなしだった, "I was on my feet all day"; 負けっぱなし, "losing every time".
It works like a noun: っぱなしにする, っぱなしだ, っぱなしで. You'll hear it most from parents and flatmates complaining about lights, taps and doors.
Examples
電気をつけっぱなしで寝てしまった。
I fell asleep with the light left on.
水を出しっぱなしにしないで。
Don't leave the water running.
窓を開けっぱなしにして出かけた。
I went out and left the window open.
今日は一日中立ちっぱなしだった。
I was on my feet all day today.
服を脱ぎっぱなしにしないで。
Don't leave your clothes lying where you took them off.