“42” is a quotation from the science-fiction novel Hitchhikers Guide to the Galaxy by the famous Douglas Adams.

In his book the number 42 is the “answer to the meaning of life, the universe, and everything” - calculated by a very big computer over a very very long time:

“All right,” said Deep Thought. “The Answer to the Great Question…”

“Yes..!”

“Of Life, the Universe and Everything…” said Deep Thought.

“Yes…!”

“Is…” said Deep Thought, and paused.

“Yes…!”

“Is…”

“Yes…!!!…?”

“Forty-two,” said Deep Thought, with infinite majesty and calm.” Douglas Adams The Hitchhiker’s Guide to the Galaxy quoted from GoodReads

Software developers like to use 42 as magic number, that’s a number without any real significance, which you could replace by any other number. Of course the name arc42 wouldn’t sound half as cool if we had choosen another number…