unplumbable

English

Etymology

From un- +‎ plumbable.

Adjective

unplumbable (not comparable)

1. Not plumbable; unfathomable.
   • 1988, David Quammen, The Flight of the Iguana:

     Either because it was a less toxic species, or because by now Doug was growing immunized, or for some other unplumbable reason, in this case the effects were no worse than those from a bee sting.