A person who waits around to steal your lover from you at the first opportunity.
The cuckoo, who usually plays the role of being a "friend", is likely to be a friend of your significant other's best friend's partner. They use the convenience of "let's go out as s foursome" as part of their swooping technique.
Just as the real cuckoo waits for an opportunity to get into another bird's nest, then throws out their eggs and takes it over as their own. The cuckoo swoops in, and starts dating your partner.
"I knew that Dave bloke was sniffing around and wanted to be more than her friend. Fucking cuckoo stole my nest!"