unplumbable en · ADJ
Etymology
From un- + plumbable.
Meanings
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(not-comparable) Not plumbable; unfathomable.
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.
1988, David Quammen, The Flight of the Iguana: