be a gooseberry
From Longman Dictionary of Contemporary Englishbe a gooseberrybe a gooseberry (also play gooseberry British English) informalWITH to be with two people who are having a romantic relationship and who want to be alone together → gooseberryExamples from the Corpusbe a gooseberry• Other new season good fruit buys are gooseberries, redcurrants and blackcurrants.