"a nonsensical or paradoxical question posed to a Zen student as a subject for meditation, intended to help the student break free of reason and develop intuition in order to achieve enlightenment" (Collins English Dictionary – Complete and Unabridged, 12th Edition 2014. S.v. "koan." Retrieved August 7 2018 from https://www.thefreedictionary.com/koan)

It seems quite nonsensical, why not?... This is the best explanation so far ;-)