This is a recent cockney rhyming slang word for Nick - which is in itself slang for police station or prison cell. ie SHOVEL and PICK (nick). A police officer might say "You're nicked" when arresting someone.
noun a person who has an excessive interest in or admiration of their spade shaped heads.
"shovels who think the world revolves around them and their not so pretty heads"
Omg she is such a shovel.
Did you see that girl, she is nothing but a shovel