下敷き
したじき desk pad; sheet of plastic (or cardboard, felt, etc.) placed under writing paper
Why it means this
下 (under) + 敷き (a spread sheet) — a stiff sheet slipped under writing paper; 〜を下敷きにする, to base a work on an earlier model.
Breakdown
Each character links to its full entry — origin story, stroke order, and readings.