lightbulb

Etymology 2
, as though the third person is a light bulb in the background.

Noun

 * 1)  An uninvited third person joining a couple's date who prevents or hinders romantic interactions.
 * 2) * 2014, carolcraves, "", Twitter, 31 January 2014:
 * Being an extra big light bulb and eating with two cousins and their husband and fiance.
 * 1) * 2014, carolcraves, "", Twitter, 31 January 2014:
 * Being an extra big light bulb and eating with two cousins and their husband and fiance.