Origin and history of overweening

"be of the opinion, have the notion" (archaic), Middle English wenen, from Old English wenan "to fancy, imagine, believe; expect, hope," from Proto-Germanic *wenjan "to hope" (source also of Old Saxon wanian, Old Norse væna, Old Frisian wena, Old High German wanen, German wähnen, Gothic wenjan "to expect, suppose, think"), from *woeniz "expectation," from PIE root *wen- (1) "to desire, strive for."

Very common in Middle English, archaic since 17c. Also as a noun from Old English, "doubt, conjecture, opinion, belief."

word-forming element meaning variously "above; highest; across; higher in power or authority; too much; above normal; outer; beyond in time, too long," from Old English ofer (from PIE root *uper "over"). Over and its Germanic relations were widely used as prefixes, and sometimes could be used with negative force. This is rare in Modern English, but compare Gothic ufarmunnon "to forget," ufar-swaran "to swear falsely;" Old English ofercræft "fraud."

In some of its uses, moreover, over is a movable element, which can be prefixed at will to almost any verb or adjective of suitable sense, as freely as an adjective can be placed before a substantive or an adverb before an adjective. [OED]

Among the old words not now existing are Old English oferlufu (Middle English oferlufe), literally "over-love," hence "excessive or immoderate love." Over- in Middle English also could carry a sense of "too little, below normal," as in over-lyght "of too little weight" (c. 1400), overlitel "too small" (mid-14c.), oversmall (mid-13c.), overshort, etc.