of, pertaining to, or along a diameter.
in direct opposition; being at opposite extremes; complete: diametrical opposites; a diametrical difference.
Also, diametric.

1545–55; < Greek diametrik(ós) (diámetr(os) diameter + -ikos -ic) + -al1

diametrically, adverb
undiametric, adjective
undiametrical, adjective
undiametrically, adverb
Dictionary.com Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
diametrically (ˌdaɪəˈmɛtrɪkəlɪ)
completely; utterly (esp in the phrase diametrically opposed)

Collins English Dictionary - Complete & Unabridged 10th Edition
2009 © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
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Word Origin & History

1630s, "completely" (opposed, contrary, etc.), from diametric (q.v.). Mostly in figurative use; the two points that mark the ends of a line of diameter across a circle are opposite one another.
Online Etymology Dictionary, © 2010 Douglas Harper
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Example sentences
Consider how the same word is used in nearly diametrically-opposed senses by
  the different ends of the political spectrum.
It would be difficult to find an officer more diametrically opposed to the
  popular conception of a detective.
It is diametrically opposed to decency, intelligence, and fairness.
But this time the two markets appear to have been rising for diametrically
  opposed reasons.
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