From the manual entry:
Using an algorithm suggested by S. C. Johnson, the Unix ora-
cle simply reads a question from the standard input (up to
an EOF) and hashes the individual characters in combination
with other indicia which happen to be lying around the sys-
tem. The resulting value is used as the seed of a random
number generator which drives a simulated coin-toss divina-
tion. The answer appears on the standard output. I was always convinced that the presence of the word 'indicia' indicated (pun
intended) that Dennis Ritchie had his hand in it. Who in the Unix world today
writes, would even be able to write, a manual entry like that.