From Urban Dictionary: "The exponentially irresistible urge to laugh and giggle in inappropriate situations, such as when asked to take a moment of silence in honor of someone or something, or a solemn ceremony like a wedding or funeral. The harder one resists the urge to laugh, the funnier the idea of what would have happened HAD you laughed becomes, resulting in an even stronger urge to laugh. Coined by the British sitcom Coupling."
Coupling ran on BBC2 from 2000-2004. The Episode in question is from Series 1, Episode 3: "Sex, Death and Nudity", which aired in May 2000.