To play gooseberry

to be an unwanted third person when two people (usually in a romantic relationship) want to be alone

e.g. I am going to the cinema with my boyfriend tonight to see a romantic comedy. Unfortunately his sister insists on coming and playing gooseberry.

The idiom comes from when a chaperone had to escort an unmarried couple. If they were outdoors, the escort would commonly pretend not to be noticing the couple and to be looking at flowers or picking wild fruit; so it was firstly “picking gooseberry” then became playing gooseberry.
The modern American equivalent is playing third wheel. The reasoning is that if you have a two wheeled vehicle such as a bicycle, a third wheel is not needed or wanted!