The phrase "massive coup" refers to a sudden and successful overthrow of a government or a prominent figure in a very large and significant way. Full definition
If a deal were to go through it would be an absolutely massive coup for the club, of course, and you know Manchester United will be exploring any possibility of a move. (independent.co.uk)
These Masses were specifically requested by individuals who have publicly voiced their opposition to Catholic teaching on homosexuality for decades, and to secure such diocesan - approved Masses has undoubtedly been a massive coup for them. (faith.org.uk)
I think getting Javi martinez on loan would be a massive coup. (justarsenal.com)