thack
The weatherproof outer layer of a roof, often specifically thatch.
countable, uncountable
This outer layer was generically known as 'thack', but, owing to the fact that the vast majority of buildings in early times were covered with a thacking of straw or some similar material, 'thatch' gradually acquired its modern restricted significance of straw, or reed, heling.
Where it comes from
From Middle English thak, thakk, thakke, from Old English þæc, from Proto-West Germanic *þak, from Proto-Germanic *þaką, from Proto-Indo-European *teg-. Cognate with Dutch dak, Low German Dack, Danish tag (“roof”), German Dach (“roof”), Old Norse þak (“thatch, roof”). Akin to Latin toga (“garment”) and Ancient Greek στέγος (stégos, “roof”). See also thatch and deck.
Adapted from the Wiktionary entry thack (CC BY-SA 4.0).