Origin:
L, neut. of arcanus, shut in, hidden < arcere, to enclose: see exercise
See arcanum in American Heritage Dictionary 4
nounpl.ar·ca·na(-nə) or ar·ca·nums
A deep secret; a mystery.
often arcana Specialized knowledge or detail that is mysterious to the average person: “knows the arcana of police procedure and the intricacies of litigation”(George F. Will).