The Collaborative International Dictionary
Illapsable
Illapsable \Il*laps"a*ble\, a. [Pref. il- not + lapsable.] Incapable of slipping, or of error. [R.]
Morally immutable and illapsable.
--Glanvill.
Wiktionary
illapsable
a. Incapable of lapse or error.