The word 'thatch' originates from the Old English þæc, which simply meant "roof" or "covering." This term is part of a widespread Germanic family, with cognates like Old Norse þak and German Dach, all pointing back to a Proto-Germanic root þaką. This root's core concept was a protective layer or cover, specifically for the top of a building. Over time, as roofing materials diversified, the English term specialized to refer specifically to roofs made from dried vegetation like straw or reeds, a method common in medieval and rural architecture. Thus, the word evolved from a general term for "roof" to a more specific one denoting both the material and the technique of thatched roofing.

The word is a simplex form. It is derived from Old English þæc, meaning "roof, thatch, cover." The Old English root is related to the Old Norse þak, the Dutch dak, and the German Dach, all meaning "roof." Its functional role is as a base noun and verb.