The Collaborative International Dictionary
Apagogic
Apagogic \Ap`a*gog"ic\, Apagogical \Ap`a*gog"ic*al\, a.
Proving indirectly, by showing the absurdity, or
impossibility of the contrary.
--Bp. Berkeley.
Wiktionary
apagogic
a. prove indirectly, by showing the impossibility, or absurdity of the contrary.