下敷き (shitajiki: (1) mat, desk pad; (2) pinned under, crushed beneath) under + to spread
You may have noticed a significant schism in the two types of meanings here. The first group includes tangible objects. The second type seems to be a metaphorical extension (most likely created by someone crushed under a mat at some point).
This second meaning often crops up in the phrase 下敷きになる (shitajiki ni naru: to be squashed by, be pinned under).
A sample sentence:
彼は地震でコンクリートの破片の
下敷きになった。
Kare wa jishin de konkurīto no hahen no shitajiki ni natta.
The earthquake buried him under concrete fragments.彼 (kare: he)
地震 (jishin: earthquake) earth + to shake
破片 (hahen: fragments) to break + fragment
