I was walking on a hillside, alone, one bright summer day, when suddenly there came into my head one line of verse — one solitary line — “For the Snark was a Boojum, you see.” I knew not what it meant, then: I know not what it means, now; but I wrote it down: and, some time afterwards, the rest of the stanza occurred to me, that being its last line: and so by degrees, at odd moments during the next year or two, the rest of the poem pieced itself together, that being its last stanza.
Source: Lewis Carroll in Alice on Stage, The Theatre, April 1887.
See also: http://kellyrfineman.blogspot.de/2007/02/snark-was-boojum-you-see-poetry-friday.html
That walk over the hills near Guildford took place on 1874-07-18. I think that leaving such a nice origination story to his readers is part of Carroll’s skillful marketing of his Snark ballade.
Oliver Sturm, who translated The Hunting of the Snark into German (Die Jagd nach dem Schnatz. 1996, ISBN 978-3-15-009433-4, p. 85) called that a “lime twig for critics” (“Leimrute für Kritiker”).
I don’t think that Carroll really misleads his readers when he said “I know not what it means”. He just made his poem as ambiguous as possible. The motive: Widening the interpretation space of his Snark poem. In case his readers (like me) think they have discovered some obfuscated meaning, it is the reader (again like me) who can be hold responsible for her or his interpretation, not the author. So, as for my interpretations, there still is the possibility that I am misleading myself. (If that is the case, than at least I cannot be blamed for having published spoilers.)
This is why the Snark hunt never will end.