Humbug (exclamation)

Humbug was an exclamation the Ninth Doctor used after Charles Dickens refused to participate in Gwyneth's séance. (TV: The Unquiet Dead)

Behind the scenes
The use of "Humbug" in The Unquiet Dead is a reference to Charles Dickens, whose character,, is widely known for saying it in both his original novel of A Christmas Carol and various adaptations.