'Bucca' is the Cornish name for spirits that live in the sea and down mines. There are two types of these 'Pobell Vean' - little people or faieries: 'Bucca Gwidden' - White (good) Bucca, and 'Bucca Dhu' - the bad Black Bucca.
The good Buccas direct Cornish tin miners to good lodes (good places to mine tin or copper) with their characteristic tapping. The bad would maliciously trap miners and cause havoc underground. The Sea Bucca is thought to once have been a sea god: fishermen left a 'dega' - one tenth - of their catch on the shore to keep him sweet and keep the fish biting.
All people were expected to join the effort to keep Bucca sweet: on harvest days some bread was left out and beer spilled onto the ground to feed him, and ensure good luck for the next year. Bucca was, and still is, often used to warn sleepless children that if they did not behave, "Bucca would come and carry you off". This results from 'Bucca' also being a word used to describe a ghost.
There is a famous story, that of Tom Trevorrow, who did not respect the 'knockers' (pronounced knackers), i.e. the Bucca. The story goes that when Tom Trevorrow was working down the mine, he heard some Buccas above him. he told them to 'be quiet and go away' (This is not good, bearing in mind one is supposed to respect the knockers and keep them sweet). Upon saying this, a shower of stones fell onto him, giving him a great fright. Later, when eating his Cornish Pasty for dinner, he heard voices saying
"Tom Trevorrow, Tom Trevorrow,
Leave some of thy fuggan for the Bucca
Else bad luck shall come tomorrow!"
So Tom took no notice and ate everything (again, a bit stupid, as it is traditional for a miner to leave the crust of his pasty for bucca), on which he heard
"Tommy Trevorrow! Tommy Trevorrow!
We'll send thee bad luck to-morrow;
Thou old curmudgeon, to eat all thy fuggan,
And leave not a 'didjan' for bucca."
And after this he had such persistant bad luck he had to leave the mine and seek worse work elsewhere.
Nowadays to call a person a "Great Bucca" generally means an idiot, rather than a mischief-maker.