The story
Catchphrase of Sonny the Cuckoo Bird, the cartoon mascot of the General Mills breakfast cereal Cocoa Puffs: "I go cuckoo for Cocoa Puffs!" See cuckoo (“crazy”).
Etymology from Wiktionary, CC BY-SA 4.0.
cuckoo for Cocoa Puffs
Today it means: Crazy, insane, irrational. Full definition →