vindication (n.)

1540s, "a word," a sense now obsolete, from Late Latin dictionem (nominative dictio) "a saying, expression; a word; kind of delivery, style," noun of action from past-participle stem of Latin dicere "to say, state, proclaim, make known, allege, declare positively" (source of French dire "to say"), which is related to dicare "to talk, speak, utter, make speech; pronounce, articulate," both from PIE root *deik- "to show," also "pronounce solemnly." The meaning "manner of saying," especially in reference to the choice of words, is from 1700.

Latin dicere and dicare are presumed to have been originally the same word. De Vann writes that "the verb dicāre may well have been backformed from compounds in -dicāre." The basic sense in both is "to talk, speak, declare." They seem to have divided, imperfectly, the secondary senses between them: dicere "to say, state, proclaim, make known, allege, declare positively; plead (a case);" in religion, "to dedicate, consecrate," hence, transferred from the religion sense, "give up, set apart, appropriate;" dicare "to talk, speak, utter, make speech; pronounce, articulate; to mean, intend; describe; to call, to name; appoint, set apart."

1843, usually said to be from Latin vim, accusative of vis "strength, force, power, vigor, energy," from Proto-Italic *wis-, traditionally from PIE root *weie- "to go after, pursue with vigor or desire," with noun derivatives indicating "force, power" (see gain (v.)) and related to the root of virile. But de Vaan seems to have doubts ("more easily explained from an original root noun"), and based on the early uses OED suggests the possibility that the English word is of "a purely inventive or interjectional origin."