A termagant is a shrewish woman. Because, patriarchy. Grrr.
Okay, sorry. Actually, termagant only started being applied to women around the 16th century. Before that it was a name given to a god which Christians believed Muslims worshipped (for various reasons which mainly involve Christians being confused about every other religion). By Shakespeare’s time a ‘termagant’ had become a theatrical archetype for a ranting, bullying type (see ‘Henry IV, Part I’ for an example: ‘that hot termagant Scot’). And probably because the termagant often wore long robes, and because all the parts were played by men anyway (grrr again) audiences starting thinking of them as female. By the late 17th century this was firmly entrenched – Thomas Shadwell's play ‘The Squire of Alsatia’ had a character called Mrs Termagant who’s described as a ‘furious, malicious, and revengeful woman’.
Termagant still gets used these days, and actually turned up fairly recently in an equal opportunity insult (yay!). In 2008, the Australian politician Kim Beazley called his opponent Tony Abbott a termagant.