A groundsheet is the waterproof layer between the tent and the earth. Lay the groundsheet, a sewn-in groundsheet, the groundsheet leaked.
A sewn-in one is joined to the tent walls and keeps insects out, which is why nearly all modern tents have one. Any part of it left sticking out beyond the flysheet will channel rain underneath you.
1noungroundsheetThe waterproof layer between the tent and the earth.
“We spread the groundsheet on the grass before pitching the tent on top of it.”
groundsheet is a sheet.
Ground plus sheet, from the canvas sheet spread on the ground before floors were built into tents.
From ground + sheet.
Etymology from Wiktionary, CC BY-SA