Fetching details from the dictionary...

flat; even; level
plain (language, words, etc.); simple
この石は扁たい形をしていて水切りに最適だ。
This stone has a flat shape, making it ideal for skipping.
箱の中身は扁たかったので隙間ができた。
The contents of the box were flat, so a gap formed.
この容器は見た目ほど扁たくない。
This container is not as flat as it looks.