The Collaborative International Dictionary
Incogitable
Incogitable \In*cog"i*ta*ble\, a. [L. incogitabilis; pref. in-
not + cogitabilis cogitable.]
Not cogitable; inconceivable.
--Sir T. More.
Douglas Harper's Etymology Dictionary
incogitable
"unthinkable," 1520s, from Late Latin incogitabilis, from in- "not" (see in- (1)) + cogitabilis, from stem of cogitare (see cogitation).
Wiktionary
incogitable
a. Not cogitable; inconceivable.