baring the shoulder, strip to waist
袒 centers on exposing the upper body by removing or lowering clothing: baring a shoulder or stripping to the waist.
袒 combines 衤 (clothing) with 旦, which likely contributes the sound. The character expresses the idea of baring the body by removing or adjusting clothing.
The clothing radical 衤 beside 旦 (dawn) suggests stripping off clothes as the day begins, baring the shoulder or waist.
For タン, imagine getting a tan by stripping to the waist: tan -> タン, and your shoulder is bare.
friendship; allegiance; support