diametric adj.

late 14c., in geometry, "chord of a circle or sphere which passes through its center; the length of a diameter," from Old French diametre, from Latin diametrus, from Greek diametros (gramme) "diagonal of a circle," from dia "across, through" (see dia-) + metron "a measure" (from PIE root *me- (2) "to measure").

compound adjectival word-forming element, usually interchangeable with -ic but sometimes with specialized sense (such as historic/historical, politic/political), Middle English, from Late Latin -icalis, from Latin -icus + -alis (see -al (1)). Probably it was needed because the forms in -ic often took on a noun sense (for example physic). Forms in -ical tend to be attested earlier in English than their twins in -ic.

1630s, "completely, in an extreme degree" (with opposed, contrary, etc.), from diametrical (see diametric + -al (1)) + -ly (2). Originally and mostly in figurative use: the two points that mark the ends of a line of diameter are opposite one another. Diametrical opposition translates Aristotle's phrase for "extreme opposition."