"Bull" is being used as a
double entendre -- meaning both the animal shown
on the nickel and the other meaning of stupid or untrue talk or writing; nonsense. So, the bull is coming off the nickel and being applied to the situation at hand. The situation at hand being
nonsensical.
This obscure phrase is famously used in the Twilight Zone episode "Mr. Garrity and the Graves". Rod Serling, in the intro, uses this expression in referring to Jared Garrity, who makes a living restoring life to the dead.
"Mr. Garrity, if one can believe him, is a resurrecter of the dead - which,
on the face of it, certainly sounds like
the bull is off
the nickel." -- Rod Serling