reductio ad absurdum
Concise Oxford English Dictionary © 2008 Oxford University Press:
reductio ad absurdum /rɪˌdʌktɪəʊ ad abˈsəːdəm/
▶noun Philosophy a method of proving the falsity of a premise by showing that its logical consequence is absurd or contradictory.
– origin L., lit. ‘reduction to the absurd’.
'reductio ad absurdum' also found in these Oxford entries:

