[BRITISH] An additional person who is neither necessary nor wanted in a given situation; what we in the US would call a third wheel.
“We met girls, Hugo. One for CP, one for Olly, one for moi. Come and say hello.”
“You know how shy I am around women.”
He finds this too funny to laugh at. “Pants—on—fire.”
“Really, Fitz, no one loves a gooseberry. Who are they?”
Definition from Wiktionary, Creative Commons Attribution/Share-Alike License (via Wordnik)