The expression “a leap in the dark” means: Any undertaking the outcome of which cannot be foreseen; a venture of uncertain consequence.
At least, such are the modern interpretations, and we apply the metaphor to just about anything we have under contemplation of which the consequences cannot be determined.
But the earliest usage of which we have record gives the phrase a more sinister interpretation.
That is, in Sir John Vanbrugh’s The Provok’d Wife (1697), we find (Act V, scene 6), the words of one dying, “So, now, I am in for Hobbes voyage, a great leap in the dark.”
The allusion being that the English philosopher, Thomas Hobbes, 1588-1679, on his deathbed was alleged to have said, “Now I am about to take my last voyage, a great leap in the dark.”
