Why does Scrooge say 'Bah humbug'?

It is first referenced in "The Student, 1750-1751, ii. 41" and is deemed to mean. A hoax; a jesting or befooling trick; an imposition or a thing which is not really what it pretends to be; an imposture, a deception, fraud, sham or indeed a Deception, pretence, sham; used interjectionally a similar phrase would have been 'stuff and nonsense!'.