The story
Calque from Cantonese 電燈泡／电灯泡 (din6 dang1 paau1), as though the third person is a light bulb in the background.
Etymology from Wiktionary, CC BY-SA 4.0.
lightbulb
Today it means: An uninvited third person joining a couple's date who prevents or hinders romantic interactions. Full definition →