means The craft of covering a roof with dry vegetation such as straw, reeds, or rushes layered so that water runs off rather than soaking in.

from From the verb 'thatch,' which descends from Old English 'þeccan,' meaning 'to cover' — a sense so basic it shares deep Germanic roots with words like German 'decken' (to cover) and is a distant cousin of Latin 'tegere,' also 'to cover' (the source of 'protect' and 'tile'). The noun 'thatch' for the roofing material grew from this covering sense, and 'thatching' simply names the act of doing it. Fittingly, the word's whole family tree is about putting a lid on things.