noun
leaving on; leaving engaged
Describes the state of having left something on or engaged, such as a light, TV, or a key in a lock. Often used in complaints or reminders.
See also: っぱなし
テレビをつけっぱなしで寝てしまった。
I fell asleep with the TV left on.
鍵をつけっぱなしにし
Please don't leave the key in the lock.
っぱなし is a suffix meaning 'leaving something in a state'; つけっぱなし is a specific noun formed from つける + っぱなし.
Formed from the verb つける (to turn on, to engage) and the suffix っぱなし (leaving something as is).