Etymology
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reductio ad absurdum 
Latin, literally "reduction to the absurd." Absurdum is neuter of absurdus. See reduction + absurd. The tactic is useful and unobjectionable in proofs in geometry.
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absurdity (n.)

late 15c., absurdite, "that which is absurd," from Late Latin absurditatem (nominative absurditas) "dissonance, incongruity," noun of state from Latin absurdus "out of tune;" figuratively "incongruous, silly, senseless" (see absurd).

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-ide 

word-forming element used in chemistry to coin names for simple compounds of one element with another element or radical; originally abstracted from oxide, which was the first so classified, in which the -ide is from acide "acid."

The suffix is really -dus (-do-), the -i- repr. the orig. or supplied stem-vowel ; it occurs without the vowel in absurdus, absurd, blandus, bland, crudus, raw (crude), etc. [Century Dictionary]
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absurd (adj.)
Origin and meaning of absurd

"plainly illogical," 1550s, from French absurde (16c.), from Latin absurdus "out of tune, discordant;" figuratively "incongruous, foolish, silly, senseless," from ab- "off, away from," here perhaps an intensive prefix, + surdus "dull, deaf, mute," which is possibly from an imitative PIE root meaning "to buzz, whisper" (see susurration). Thus the basic sense is perhaps "out of tune," but de Vaan writes, "Since 'deaf' often has two semantic sides, viz. 'who cannot hear' and 'who is not heard,' ab-surdus can be explained as 'which is unheard of' ..." The modern English sense is the Latin figurative one, perhaps "out of harmony with reason or propriety." Related: Absurdly; absurdness.

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