The author of Junnus says this was a Saxon idol, and derives the word from tyr magan (very mighty); but perhaps it is the Persian tir-magian (Magian lord or deity). The early Crusaders, not very nice in their distinctions, called all Pagans Saracens, and muddled together Magianism and Mahometanism in wonderful confusion, so that Termagant was called the god of the Saracens, or the co-partner of Mahound. Hence Ariosto makes Ferrau “blaspheme his Mahound and Termagant” (Orlando Furioso, xii. 59); and in the legend of Syr Guy the Soudan or Sultan is made to say -
So helpë me, Mahòune, of might, And Termagaunt, my God so bright.
Termagant was at one time applied to men. Thus Massinger, in The Picture, says, “A hundred thousand Turks assailed him, every one a Termagant [Pagan].” At present the word is applied to a boisterous, brawling woman. Thus Arbuthnot says, “The eldest daughter was a termagant, an imperious profligate wretch.” The change of sex arose from the custom of representing Termagant on the stage in Eastern robes, like those worn in Europe by females.
“ `Twas time to counterfeit, or that hot termagant Scot [Douglas] bad paid me scot and lot too.” —Shakespeare: 1 Henry IV., v. 4.
Outdoing Termagant (Hamlet, iii. 2). In the old play the degree of rant was the measure of villainy. Termagant and Herod, being considered the beau-ideal of all that is bad, were represented as settling everything with club law, and bawling so as to split the ears of the groundlings. Bully Bottom, having ranted to his heart's content, says, “That is Ercles' vein, a tyrant's vein.” (See Herod.)
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894