coarse mat of rushes or bamboo
笪 refers to a coarse mat woven from rushes or bamboo, used as a simple floor covering or wrapping material.
笪 combines 竹 (bamboo) with 旦, which likely contributes a phonetic element. The character originally denoted a coarse mat made from bamboo or rushes.
The bamboo top 竹 over 旦 suggests a flat, woven surface: imagine a coarse mat made of bamboo strips spread out under the morning sun (旦).
For タン, picture a tan-colored coarse mat: tan -> タン, and the mat is woven from bamboo.