gall bladder, bravery, courage
膽 centers on the gall bladder as an organ, and by extension the bravery or courage traditionally associated with it in East Asian thought.
膽 combines the flesh radical ⺼ (body part) with 詹, which likely contributes the sound. The character originally referred to the gall bladder, with the extended meaning of courage arising from traditional associations.
The flesh radical ⺼ on the left marks this as a body part. The right side 詹 looks like a container with a lid, like the gall bladder storing bile. Together they form the organ linked to bravery.
For タン, imagine a tan warrior whose gall bladder gives him courage: tan -> タン, and his bravery comes from the organ.