"I don't want to play gooseberry."

In one of the old 'Brit Coms' on TV a character says "I don't want to play gooseberry." when she is invited for a weekend trip to the country. She and a male friend were previously invited and the male friend had to cancel. Obviously she had some ambivalence but I did not understand beyond that.

From a discussion in the archives under "gooseberry." Mr. Briggs says:

To play gooseberry now means to be an unwelcome third party at a lovers' meeting. In the past it was used somewhat less specifically and meant any unwanted third party. In the old days Gooseberry was one of the many euphemisms for the Devil, who was naturally not welcome in most company.