The answer to the Ultimate Question of Life, the Universe, and Everything. Although we never really learned what the question itself was unless "What is 6x7?" has some greater as yet not understood cosmic significance.
Have you ever tried to think about stuff like the vastness of the universe or how things were initially created? It hurts my head when I try.
Speaking of 42, I found an anecdote
I thought was amusing.
Deep Thought
"Scientists in Cambridge spent three years calculating
one of the fundamental keys to the universe - The Hubble
Constant [the velocity at which a typical galaxy is receding
from Earth divided by its distance from Earth] that determines
the age of the universe. This process mirrored a passage in
[Douglas Adams's] cult science fiction novel and radio series
'The Hitchhiker's Guide to the Galaxy' in which an alien race
programs a computer called Deep Thought to provide the
ultimate answer to understanding life and the universe. "In
the novel, seven and a half million years later Deep Thought
comes back with the result, 42.
"In an extraordinary coincidence when the Cambridge scientists
finally calculated the Hubble Constant they found the answer
was also 42."
["It caused quite a few laughs when we arrived at the figure
42," Dr Keith Grange later recalled, "because we're all great
fans of The Hitchhiker's Guide!" (Sadly, estimates of the Hubble
Constant now range between 57 and 78 kilometers per second
per megaparsec.)]