Old English að "oath, judicial swearing, solemn appeal to deity in witness of truth or a promise," from Proto-Germanic *aithaz (cf. Old Norse eiðr, Swedish ed, Old Saxon, Old Frisian eth, Middle Dutch eet, Dutch eed, German eid, Gothic aiþs "oath"), from PIE *oi-to- "an oath" (cf. Old Irish oeth "oath"). In reference to careless invocations of divinity, from late 12c.
a solemn appeal to God, permitted on fitting occasions (Deut. 6:13; Jer. 4:2), in various forms (Gen. 16:5; 2 Sam. 12:5; Ruth 1:17; Hos. 4:15; Rom. 1:9), and taken in different ways (Gen. 14:22; 24:2; 2 Chr. 6:22). God is represented as taking an oath (Heb. 6:16-18), so also Christ (Matt. 26:64), and Paul (Rom. 9:1; Gal. 1:20; Phil. 1:8). The precept, "Swear not at all," refers probably to ordinary conversation between man and man (Matt. 5:34,37). But if the words are taken as referring to oaths, then their intention may have been to show "that the proper state of Christians is to require no oaths; that when evil is expelled from among them every yea and nay will be as decisive as an oath, every promise as binding as a vow."