Witherspoons

There is something that puzzles me. Daphne calls people "Witherspoons" a few times in the movie. When she is going to the US, she says "I think the boat is teaming with Witherspoons". When she learns Ellen has been speaking with a Canadian couple, she says "with these absolute Witherspoons?", etc.
What are these "Witherspoons" she is referring to? What does she mean? Does anybody know?
Thank you.