a Boojum was, ostensibly, a particularly dangerous kind of snark. Lewis Carroll invariably described the snark as "unimaginable".

 Originally Posted By: Lewis Carroll
But oh, beamish nephew, beware of the day, If your Snark be a Boojum! For then You will softly and suddenly vanish away..
[EA]

The dreadful Boojum of Nothingness. - W. H. Auden