context: "[L]evity can sometimes be the prelude to indifference. And indifference, in turn, the viaticum for disaster."
definition from Merriam-Webster: a) an allowance (as of transportation or supplies and money) for traveling expenses; b) provisions for a journey
my thoughts: I guess the "indifference" makes one oblivious to the signs of impending "disaster", thus making that disaster unavoidable. If you don't take things seriously, if you blithely disregard, when the bad overtakes you, you will be surprised, of course, but you had your chance. Indifference as "viaticum", then, is indifference feeding the disaster, bringing it forth.
source:The Worst Intentions a novel by Alessandro Piperno, translated by Ann Goldstein