word-forming element meaning "beyond" (ultraviolet) or "extremely" (ultramodern), from Latin ultra- from ultra (adv. and prep.) "beyond, on the other side, on the farther side, past, over, across," from PIE *ol-tero-, suffixed form of root *al- "beyond." In common use from early 19c., it appears to have arisen from French political designations. As its own word, a noun meaning "extremist" of various stripes, it is first recorded 1817, from French ultra, shortening of ultra-royaliste "extreme royalist."
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."
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Harper, D. (n.d.). Etymology of ultra vires. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/ultra vires