undiagnosably

Etymology tree
Proto-Indo-European *né
Proto-Indo-European *n̥-
Proto-Germanic *un-
Proto-West Germanic *un-
Old English un-
Middle English un-
English un-
Ancient Greek δῐαγῐγνώσκω (dĭagĭgnṓskō)
Ancient Greek -τις (-tis)
Ancient Greek -σῐς (-sĭs)
Ancient Greek δῐᾰ́γνωσῐς (dĭắgnōsĭs)der.
Latin diagnōsisder.
English diagnosisbf.
English diagnos(e)
Proto-Indo-European *-dʰlom
Proto-Indo-European *-dʰlis
Proto-Italic *-ðlis
Latin -bilis
Latin -ābilis
Old French -ablebor.
Middle English -able
English -able
English diagnosable
English undiagnosable
Middle English -ly
English -ly
English undiagnosably
From undiagnosable + -ly.