Reading
〜っ{放|ぱな}し
implies abandoning something, leaving it in an improper state (usually on). It has the nuance that it was left behind as an oversight, or just neglected. It then becomes a 形容動詞 (な-adjective). It comes from the aux. verb 放す, together with a 連用中止法 (i-stem).
YouTubeをつけっぱなしにしなければ眠れない人は、結構いる。 People who who cannot sleep unless a YouTube video is playing in the background, there exists a fair number of them!
窓を開けっ放しだったので、蚊がいっぱい入ってしまった。 As the window was left open, a lot of mosquitoes got in.
水を出しっぱなしにないでください。 Please don't leave the water running
Note that unlike てある, it does NOT change the particle to が for transitive verbs. it respects the original particle.