"ascertaining of the position of a ship by measurement of the distance run" (without observation of heavenly bodies), 1610s, perhaps from nautical abbreviation ded. ("deduced") in log books, but it also fits dead (adj.) in the sense of "unrelieved, absolute."
c. 1200, "straight down, right down, perpendicularly," from down (adv.) + -right. The meaning "thoroughly, completely, utterly," often merely emphatic, is attested from c. 1300. As an adjective, "complete, absolute," from 1560s. Old English had dunrihte "downwards." The inverted form right-down is attested 17c.
1755 as a term in philosophy, "a way of thinking which explains phenomena by the assumption of two independent and absolute elements," from French dualisme (1754); see dual + -ism. Theological sense of "doctrine of two independent divine beings or eternal principles" is by 1847. General sense of "division into two" is by 1831.
late 14c., "legal dissolution of the bond of marriage," from Old French divorce (14c.), from Latin divortium "separation, dissolution of marriage," from divertere "to separate, leave one's husband, turn aside" (see divert). Not distinguished in English from legal separation until mid-19c. Extended sense of "complete separation, absolute disjunction" is from early 15c.
early 15c., maxime, "an axiom, statement of a self-evident truth," from Old French maxime, from Late Latin maxima, shortened from phrases such as maxima propositio (Boethius), maxima sententarium "axiom," literally "greatest or chief premise, greatest among propositions" (one which is general and absolute), from fem. of maximus "greatest," from PIE *mag-samo-, superlative form of root *meg- "great."
The modern meaning "summary statement of an established or accepted proposition serving as a rule or guide, a proposition ostensibly expressing some general truth" is from 1590s.