play gooseberry

play gooseberry {UK, humorous} (to be with two people who are having a romantic relationship and who would prefer to be alone) — быть третьим лишним

Example: Yes, thank you, I'd love to go to the cinema, if you two are sure you don't mind me playing gooseberry. — если я для вас не стану третьим лишним;

"play gooseberry", or to be an uncomfortably superfluous 3rd person between two lovers, goes back to the early 19th century and may have originated in the notion of a chaperone (ostensibly) occupying herself with picking gooseberries while the couple being chaperoned did what they were doing (gooseberry-picker was an early 19th century term for a chaperone).

The Oxford English Dictionary confirms this use of "gooseberry-picker", so this explanation looks quite convincing.

[третий лишний]