This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
The exact historical development of 笫 is uncertain. It visually contains 竹 (bamboo) on top, but the lower component's origin is unclear.
With bamboo 竹 on top and a lower shape resembling a mat or bed frame, imagine a bamboo mat used for sleeping or sitting.
For シ, picture a sheet of bamboo matting: sheet -> シ, and the bamboo mat is spread on the floor.