Character is a pictogram of a well or mine with cinnabar inside. It can also indicate "red cinnabar" used in dyeing clothes (e.g., 彤). The original version is 𠂁.

Schuessler (2007) suggests a Kam-Tai origin, comparing it to Proto-Kam-Sui *h-lanꟲ (“red”). 矸, as in 丹矸, may be a variant; see there for more.

Alternatively, it may be from Proto-Sino-Tibetan *t(j)a-n ~ *tsa-n (“red”), whence Burmese တာ (ta, “very red”) (Matisoff, 2003; Hill, 2014); however, STEDT currently does not compare this to 丹.

Possibly related to 旃 (OC *tjan, “a plain red flag”), as well as 袒 (OC *daːnʔ, “to bare; to strip oneself naked to the waist”), as skin color is often associated with "red". See these entries for more.