re·duc·ti·o ad ab·sur·dum 
(rĭ-dŭktē-ō ăd əb-sûrdəm, -zûr-, -shē-ō)
(rĭ-dŭktē-ō ăd əb-sûrdəm, -zûr-, -shē-ō)n. pl. re·duc·ti·o·nes ad absurdum (-ōnēz, -nās)
Disproof of a proposition by showing that it leads to absurd or untenable conclusions.
[Medieval Latin reductiō ad absurdum : Latin reductiō, a bringing back, reduction + Latin ad, to + Latin absurdum, absurdity, from neuter of absurdus, absurd.]