reductioad absurdum

re·duc·ti·o ad ab·sur·dum (rĭ-dŭkʹtē-ō ăd əb-sûrʹdəm, -zûrʹ-, -shē-ō) n. pl. re·duc·ti·o·nes ad absurdum (-ōʹnēz, -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.]

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